Three studies were performed to evaluate the prevalence of seasonal changes in the general population: a telephone survey in Montgomery County, a mail-out survey at four latitudes along the East Coast, and a survey in doctors' offices at three out of the four latitudes mentioned above. All studies used the Seasonal Pattern Assessment Questionnaire (SPAQ) previously developed by our group. The general population studies showed that seasonal changes are a problem form approximately one quarter of the population. At the further northern latitudes more people complain about winter, whereas in the most southern latitude (Sarasota, Florida) complaints about summer were commoner. We derived case finding criteria for SAD, subsyndromal SAD (S-SAD) and summer SAD from the SPAQ, and on the basis of these criteria, found the incidence of these three conditions to vary between 1.4-8.9%, 2.6-11% and 0.5- 1.2%. There was a significant correlation between the prevalence of S-SAD and latitude, and S-SAD plus SAD, and latitude. The latter figure is of particular interest as it encompasses those individuals who are likely to benefit from enhanced environmental lighting. Extrapolating from the prevalence-latitude curve we estimate that this last combined group may number as many as 36 million people in the continental United States.