Prevalence figures of depression usually show a sex ratio for males:females of 1:2. This unequal ratio has been examined by varying the time periods for the calculation of prevalance rates and the criteria for case definition. Based on results of a 20-23 yr-old cohort of 591 males and females studied in Switzerland, some evidence was found to support the hypothesis that an unequal sex ratio is partially due to unequal reporting of depressive symptoms and to differential forgetting, perhaps linked with the male role in society. It is not yet possible to extend the results of this study to older cohorts. Further research would be desirable.