The study is based on sibships with multiply afflicted members derived from a family study of consecutively admitted probands with catatonic schizophrenia. As shown recently, the clinical subtype of periodic catatonia, as defined by Leonhard, is compatible with a major gene effect and genetic anticipation; that is, the age of illness onset of the probands is significantly earlier than that of their parents. In the present study, 83 probands with the clinical subtype of periodic catatonia had 26 afflicted siblings that were distributed among 23 families. We analyzed sex-concordance and pseudoautosomal inheritance patterns. Stratifying the 26 afflicted siblings by sibship size and by the proband's sex, we did not find unusual sex-concordance rates in sibships afflicted with periodic catatonia. Further, there was no association between sex concordance and maternal or paternal origin of the disease. Thus, our results provide strong evidence against pseudoautosomal inheritance or sex-linked transmission in affected sibships in the obviously familial schizophrenic subtype of periodic catatonia.