The phenotypic indicators of the genomic imprinting model were applied to clinical psychopathology data on 100 bipolar (BP) I probands and their families. The paternal transmission was associated with a significantly younger age of onset of the BP illness in probands and a higher rate of affective disorders in first- and second-degree relatives. The effect of the sex of the transmitting parent on age of onset in probands decreased but remained significant when controlling for the effect of the probands' age at investigation. Probands' sex had no significant influence on their age of onset. The severity of the BP illness in probands in terms of number of illness episodes and annual frequency was not influenced by the sex of the transmitting parent.