A specific family history or genetic background may be used to distinguish valid subgroups in patients who show similar symptoms. Also, a familial background may predict differences in other characteristics, i.e. course of illness, response to treatment or biological characteristics. Two hundred and fifty-one bipolar patients were separated according to their family history, 20 with a family history of mania with or without depression, 86 with a family history of depression only, and 145 with a family history of neither mania nor depression. The group that had a family history of mania was notable in that it showed more episodes of affective illness and was more likely to be readmitted to hospital. This difference in course suggests a familial association with multiple episodes and mania. In other respects than in course of illness, the groups separated by family history were similar.