The phenotypic indicators of genomic imprinting were applied to the familial psychopathology data collected through the family history method about 886 adult relatives of 65 manic-depressive probands directly investigated. The probands and their relatives were diagnosed according to DSM-III/DSM-III-R criteria. A first analysis of the age at onset of the BP illness by affective status of the probands' parents suggested that the BP disorder begins about 8 years earlier in the probands whose father was affectively ill (13.84% cases) than in the probands whose mother was affectively ill (24.6% cases) (t = -3.29, P < .004). When controlling this result for the effect of the probands' sex, its statistical significance decreased. The severity of the BP illness seemed also to be influenced by the affective status of the probands' father but only when assessing the probands' illness severity over a long time period and taking into account their psychosocial functioning; the number of manic and depressive hospitalized and non-hospitalized episodes as a single measure of the BP disorder severity as well as the morbidity risk in the first degree relatives of the probands did not significantly differentiate the patients whose disorder was transmitted by the father/paternal side as compared with the patients who inherited the BP disorder from the mother/maternal side.