Recent reports that some influenza epidemics may be followed by a transient increase in the births of schizophrenic patients have led to the hypothesis that maternal viral infections contribute to the aetiology of schizophrenia. It is well known that respiratory viral infections are frequently brought into the home by young children. We tested the predictions that the risk of schizophrenia is decreased in first-born children, and increased in individuals who had siblings of a young age while in utero, using data from a Swedish family study. Our results are consistent with these predictions. In particular, having siblings three to four years older was associated with a significantly increased risk of schizophrenia, even after allowing for birth order, sibship size, and other potential confounders. If replicated, these results provide indirect support for the maternal viral infection hypothesis, although there are alternative explanations.