Forty-five patients with winter Seasonal Affective Disorder (SAD) completed the Personality Diagnostic Questionnaire-Revised (PDQ-R) in a depressed state before starting therapy (light or medication), and again 14 weeks later when the scores on the Montgomery Asberg Depression Rating Scale (MADRS) had been reduced by an average of 74%. Prevalence and pattern of personality disorders (PDs) before treatment were similar to that found in patients with nonseasonal major depressive disorder (MDD) during a depressive episode. Number of categorical diagnoses were lower at follow-up for most PDs, but differences were not statistically significant. However, for dimensional scores (number of positive DSM-III-R criteria) significant changes were found for schizotypal, histrionic, narcissistic, and obsessive-compulsive PD, and for total dimensional score in cluster B and total score for all PDs. These changes differ from findings in similar studies of outpatients with nonseasonal MDD. Whether this indicates a specific pattern of change for patients with SAD is discussed. Multiple regression analysis showed a significant association between changes in positive PD criteria between the depressed and nondepressed state and change in depression score. No significant relationship was found between changes in PD criteria and Global Seasonality Score (GSS), a measure reflecting the sum of ratings of seasonal variation in sleep length, social activity, mood, weight, appetite, and energy level.