Background. As part of a collaborative linkage study, the authors obtained clinical and demographic data on 160 families in which more than one sibling was affected with a bipolar illness. The aim of the study was to identify clinical characteristics that had a high degree of familiality.

Method. Data on age at onset, gender, frequency of illness-episodes and proportion of manic to depressive episodes were examined to determine intra-pair correlations in affected sibling pairs. Dimension scales were developed measuring frequency and severity of lifetime mania, depression, psychosis and mood-incongruence of psychotic symptoms; degree of familial aggregation for scores on these dimensions was calculated.

Results. Sibling pairs correlated significantly for age at onset (ρ = 0·293, P<0·001); dimension scores for psychosis (ρ = 0·332, P < 0·001); and proportion of manic to depressive episodes (ρ = 0·184, P = 0·002). These findings remained significant when correcting for multiple testing. Of the other test variables; mania (ρ = 0·171, P = 0·019); incongruence dimensions (ρ = 0·242, P = 0·042); frequency of manic episodes (ρ = 0·152, P = 0·033); and frequency of depressive episodes (ρ = 0·155, P = 0·028) were associated with modest correlations but these were not significant after correction. Degree of familial aggregation was not significant for sex (κ = 0·084) or dimension scores for depression (ρ = 0·078, P = 0·300).

Conclusions. Significant but modest familial resemblance has been shown for some specific features of bipolar illness, particularly age at onset and degree of psychosis. Further research may establish the extent to which these findings are mediated by genetic and/or environmental factors.