In a family study involving 83 probands with periodic catatonia a subtype of DSM IIIR schizophrenia, we reported an age-specific morbidity risk of 26.9% in first-degree relatives with homotypical psychoses and genetic anticipation indicating a possible major gene effect. Paternal transmission was associated with a trend for a younger age at onset in probands compared to that observed in the case of maternal transmission (P = 0.099). If this can be confirmed in a larger sample and then replicated, there would be evidence for the occurrence of a parent-of-origin effect. Such an observation may indicate that a paternally imprinted locus acts on periodic catatonia. Among the non-genetic mechanisms that may modify the penetrance of the disease, paternal affection did lead to a decrease in male offspring (P = 0.007) and maternal affection showed an increased frequency of non-affected male offspring (P = 0.021). We therefore propose that parent-of-origin effects as well as prenatal mortality and psychosocial factors need further investigation in the periodic catatonia subtype of schizophrenia.