The impact of latitude on the prevalence of seasonal depression.

<bold>Background: </bold>This study sought to determine whether the prevalence of the seasonal subtype of major depression (SAD) in the community varied as a function of latitude.<bold>Methods: </bold>Random telephone numbers were generated across 8 degrees of latitude (41.5 degrees N to 49.5 degrees N) for the province of Ontario. Eight strata of 1 degree each were sampled equally throughout a 12-month period. Using a validated and structured diagnostic interview, we interviewed by telephone respondents over 20 years of age who had lived in the region for 3 years or more. We evaluated patterns of symptom change across seasons to establish a diagnosis of SAD according to DSM-IV criteria.<bold>Results: </bold>Of the 2078 households that were assessed for eligibility, 1605 (77%) completed the interview. The crude prevalence of lifetime SAD was 2.6% (95% CI, 1.9 to 3.5). There was no impact of latitude on prevalence of either major depression or the seasonal subtype across the 8 strata, although the global measure of the severity of seasonal change in mood was significantly negatively correlated with latitude.<bold>Conclusions: </bold>SAD is a common subtype of major depression in Ontario, but there is no evidence to support an increase in prevalence with increasing latitude. [ABSTRACT FROM AUTHOR]