The concentrations of sex hormone binding globulin (SHBG) were measured in the plasma of 56 men, who were 47 to 85 years of age, by time-resolved immunofluorometric assay with a monoclonal antibody. Twenty-five of the men had untreated carcinoma of the prostate and 17 had untreated prostatic hyperplasia. There were 14 healthy control subjects. SHBG levels were significantly higher in patients with prostatic carcinoma (37.6 k 8.4 nmol/l) than in those with prostatic hyperplasia (24.5 -+ 5.2 nmol/l; P < 0.05) or control subjects (14.9 t 2.8 nmol/l; P < 0.01). It is not known why SHBG levels are higher in patients with carcinoma or hyperplasia of the prostate. T like the growth and function of other organs of the male and female reproductive tracts, are under endocrine control, especially by the sex steroids. Because the most important of these for the prostate is the androgens, with dihydrotestosterone (DHT) being the most many attempts have been made to see whether or not the occurrence and the development of prostatic carcinoma result from abnormal androgenic stimulation. Currently, the results of these studies are controversial because men with prostatic cancer had plasma testosterone levels relatively lower than those of men of the same age without the d i~e a s e .~ In addition to assays of total or free hormone levels, the binding capacity of plasma proteins for steroids has been measured for these patients and for those with pathologic features of other hormone-dependent organs. Currently, however, none of these has provided results that are discriminant for carcinoma. Patients with prostate carcinoma who had not been treated had sex hormone binding globulin (SHBG) levels within the limits of normal From the Department of Urology and tBiochemistry Laboratory, Istituto S. Raffaele, Milano; and the *Consiglio Nazionale delle Ricerche, Milano, Italy.The authors thank Mr. Fabio Bravi (Centro Ricerche Italfarmaco, Cinisello, Milano) for statistical analysis of the data in this study.Address for reprints: Marco Grasso, Divisione di Urologia, Via 01-gettina 60, 20 I32 Milano, Italy.Accepted for publication January 3, 1990. when assayed by the Rosner method5 and did not have higher levels than patients with prostatic hyperplasia.6 There also are no differences in SHBG levels in relation to the stage of carcinoma or whether or not metastases exist.Recently, it was suggested that since the plasma binding globulin for steroids is not homogeneous, some method of assay might be able to show differences that others had not been able to show. The use of a suitable assay method has shown that for cancer of another hormone-dependent organ (the breast) there is an increase in steroid binding capacity that is great enough to differentiate the patients from the control subjects, and this assay has been proposed as an indicator for turn or^.^ Starting from this, we measured plasma SHBG concentrations in patients with carcinoma and hyperplasia of the prostate and in healthy control subjects by a new immunofluorometric assay t...