It is argued that those patients who switch to mania under antidepressant therapy are prone to bipolar disorder and that there is a category of mood disorders called as “bipolar III”. If clinical relevance of this category that is not included in official nosologies testified, the diagnostic and therapeutic approach to mood disorders may change. To test this hypothesis, we screened for bipolar I disorder among 138 first-degree relatives of 24 patients who had switched to mania under antidepressant therapy because of unipolar depression and then compared this figure with that among 135 first-degree relatives of 24 patients having documented bipolar I disorder. Regarding the frequency of bipolar I disorder within the family, we found no significant difference between the case and the control groups. The fact that the family history regarding bipolar I disorder among those who switched to mania under antidepressant therapy did not differ significantly from that among the control subjects suggest a genetic susceptibility to bipolarity among the cases. Our findings verify the concepts of “soft bipolarity” and “bipolar III.