In a family study involving 139 probands with chronic DSM-III-R schizophrenia, catatonic type, 83 probands met the criteria for periodic catatonia and 56 probands those for systematic catatonia according to Leonhard. In the systematic catatonias, we found a low morbidity risk of 4.6% in first-degree relatives, an early age at first hospitalization and a high prevalence of affected males. In the light of our recent report of an association between maternal gestational infection and systematic schizophrenia, male fetuses exposed to midgestational infection seem to be particularly at risk of developing systematic catatonia. Periodic catatonia with a family morbidity risk of 26.9% affected both genders with equal frequency and showed no age-at-onset differences between the genders. We found a moderate inverse relationship between early-onset probands and an increased risk in relatives of 24.1% compared to 17.8% in late-onset probands. Our findings substantiate the hypothesis that periodic catatonia is a clinically homogenous disorder with a major gene effect and an age at onset which is to a large extent genetically determined.