To determine the characteristics and time course of diagnostic conversion from unipolar depression to bipolar depression in a retrospective cohort of psychiatric outpatients, and to compare the clinical and sociodemographic profiles of the bipolar switch group against a matched group without conversion.

This is a retrospective case-control study in which outpatients newly diagnosed of unipolar depression from 1^st January 1994 to 31^st December 1999 were reviewed. Those with conversion to bipolar depression were identified and controls were matched.

Multivariate conditional logistic regression was carried out to identify predictors of bipolar switch.

88 patients out of 823 were identified with bipolar switch. The mean age at switch was 37.58 years old. The mean time interval from presentation to conversion was 58.13 months. Sodium valproate was most commonly used after switch. Bipolar switch was associated with male sex (OR 2.601, 95% CI 1.279 – 5.291, p=0.008), an earlier age at presentation of depression of <37 years old (OR 2.238, 95% CI 1.136 – 4.409, p=0.020), family history of bipolar affective disorder (OR 5.684, 95% CI 1.149 – 28.124, p=0.033) and use of 3 or above different antidepressants in the first five years after presentation (OR 2.105, 95% CI 1.082 – 4.098, p=0.028).

The incidence of bipolar switch found in this study was 10.7%. Family history of bipolar affective disorder and frequent switch of antidepressants in depressive patients presenting at a young age can be helpful guidance for identification of those at high risk for a bipolar course.