We conducted a study of 126 patients with anal and rectal squamous cell carcinoma and 372 randomly selected control subjects in the San Francisco Bay Area (CA) to test the hypothesis that these tumors are related to a history of anal intercourse, the presence of sexually transmitted diseases and other conditions of the anal area, treatment of these diseases or conditions, and history of use of cigarettes or other substances. The relative risk (RR) of cancer was elevated for men with a history of homosexual activity (RR = 12.4, P less than .001). However, after adjustment for other risk factors, this risk was reduced to 2.7 (P = .28). Risk was elevated for homosexual male patients who reported a history of genital warts (RR = 12.6, P = .03), anal fissure or fistula (RR = 9.1, P = .05), and cigarette smoking (RR = 1.9 for 20 pack-yr, P less than .001; RR = 5.2 for 50 pack-yr, P less than .001). (Pack-year is a unit of cigarette use equal to 365 packs.) There was also elevated risk for heterosexual male and female patients who reported a history of genital warts (RR = 4.4, P = .003), anal fissure or fistula (RR = 2.4, P = .03), and more than 12 episodes of hemorrhoids (RR = 2.6, P less than .001). These findings suggest that anal cancer risk is etiologically related to human papillomaviruses that cause genital warts. In addition, constant irritation, chronic inflammatory changes, and repeated epithelial regeneration that accompany noninfectious conditions may be related to risk of anal cancer. The higher risk among homosexual men is related to the higher prevalence of anal cancer risk factors for this group.

Within twin pairs, risk of PD is inversely correlated with the dose (in pack-years) of cigarette smoking. This effect is most pronounced in MZ twins, despite the high correlation for smoking. Because MZ twins are genetically identical and are similar behaviorally, this difference is unlikely to result from either genetic factors or environmental confounders. These results are compatible with a true biologic protective effect of cigarette smoking.

This pattern is consistent with a genetic cause of essential tremor. Because monozygotic concordance is not 100%, environmental factors may also play a role in the cause of the disease.

A population-based case-control study of cutaneous malignant melanoma (CMM) was conducted in 452 women with melanoma and 930 control subjects aged 25-59 years in five San Francisco Bay Area counties between 1981 and 1986. Women were interviewed in their homes with regard to history of sunlight exposure and sunburns during different periods in their lives, phenotypic and host characteristics, medical history, occupation, and demographic factors. Data were analyzed by the patients' histologic type of melanoma; 355 women were classified as having superficial spreading melanoma (SSM), 61 had nodular melanoma (NM), 13 had lentigo maligna melanoma, and 23 had other melanomas that could not be further classified upon histologic review by University of California dermatopathologists. Univariate results from analysis of factors related to sun exposure showed that the risk of all histologic types of CMM, SSM, and NM increased with increasing tendency of the subject to sunburn and with history of increased severity and/or frequency of sunburns up to age 12 years. Risk of all types of CMM and SSM also increased with increasing number of sunburns for all age groups and with lack of use of sunscreen. After adjustment for each other and for phenotypic factors, history of sunburn up to age 12 and lack of sunscreen use were the primary sun-related factors associated with an increased risk of all types of CMM and SSM, while tendency to sunburn when exposed to 1/2 hour of noontime sun and lack of use of sunscreen were related to NM. Although having frequent sunburns before age 12 and having severe sunburns before age 12 were both strongly associated with melanoma, having large numbers of sunburns during any time period from elementary school through age 30 years and having sunburns during the 10 years prior to diagnosis or interview were all associated with a doubling of risk for SSM after adjustment for other factors. These results suggest that the increased risk of melanoma related to sunburns is not confined to childhood sunburns. Maintenance of an all-year tan provided no protective effect against melanoma after adjustment for tendency to burn. No association was noted with use of fluorescent lights or exposure to sunlamps for all types of CMM, SSM, or NM.