Fifty consecutive patients with bipolar affective psychosis are reviewed, and the genetic and electroencephalographic findings set against extant hypotheses about inheritance of the disorder and the etiological homogeneity of the syndrome. Inheritance in this sample appears usually not to be X-linked. The syndrome seems etiologically heterogeneous in that some patients have an early onset and prominent genetic determinants, while in others without apparent hereditary predisposition, the illness start later and show electroencephalographic evidence of cerebral dysfunction.