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The cohort mortality experience of radiologists over a 50-year period has been compared to that of other specialists with low levels of radiation exposure. The 1920-1929 cohort of radiologists who joined the Radiological Society of North America had the highest mortality for several chronic diseases. After this early period, radiologists ranked highest only for cancer mortality. The excess risk of leukemia which was observed in the 1920-1929 and 1930-1939 cohorts has subsequently decreased. During the same period, lymphoma mortality, especially multiple myeloma, has been increasing with a significant excess of deaths appearing in radiologists who entered the specialty society between 1930-1939 and 1940-1949. A posible relationship between this finding and immunologic changes induced by radiation has been proposed.

1. Characteristics of the distribution of incubation periods of a number of infectious diseases, as reported in the literature, have been reviewed. 2. The usual frequency curve of incubation time takes the form of a logarithmic normal curve. This appears to be equally true of diseases with very short and very long incubation periods. 3. The measure of variation in incubation periods used in this study, termed the dispersion factor, is the antilogarithm of the logarithmic standard deviation. Dispersion factors for most of the diseases studied range from 1.2 to 1.5 and are independent of mean length of incubation. 4. Some epidemiological uses for a knowledge of the distribution of incubation time are discussed.

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