A proposed risk factor for schizophrenia is materno-foetal incompatibility. We tested the hypothesis that, in multiply affected families, later born children would exhibit a more severe form of schizophrenia than their older siblings. The effect of birth order on (1) severity of the worst ever episode of illness; (2) deterioration from premorbid level of functioning; (3) age of onset; (4) response to medication; and (5) illness course, was assessed in 150 sibling pairs with schizophrenia and schizoaffective disorder. We found that later birth order reduced the likelihood of regaining the premorbid level of functioning after an acute episode and was also associated with an earlier age of presentation. This study lends some support to the hypothesis that later birth order results in a more severe form of the disorder, although there are other possible explanations for our findings. Further work is needed to explore the possibility of maternal-foetal genotype incompatibility as a risk factor for schizophrenia.