Abstract Twenty-nine out of 195 bipolar/episodic schizoaffective patients were judged to be rapid-cyclers (15%). Twenty-five of the 29 were female (86%). The age-corrected morbid risk for major affective disorder was 23.5% in 179 relatives of rapid-cyclers and 31.0% in 189 relatives of matched non-rapid cyclers ( x 2 = 2.6, NS). The prevalence of rapid-cycling itself was also not different in the two groups of relatives. Rapid-cycling thus appears to arise from factors which are separable from the genetic vulnerability to bipolar illness and which do not lead to aggregation within families.