The purpose of this study was to determine if Schönlein-Henoch purpura represents an abnormal host response to microorganisms. Among 1,222 cases, representing all new Danish cases in children during the years 1977-84, there was no tendency for the cases to cluster; this means that the disease is not caused by a single, contagious agent. In a smaller sample of 281 children examined in detail, a higher number than expected attended day nursery or nursery school and 17% had received antibiotic treatment during the week prior to admission. The latter findings, together with the seasonal variation of the incidence and the activation of the immune apparatus in many cases, suggest that Schönlein-Henoch purpura may be triggered by infection with several different microorganisms, but there is no evidence that a single one such as the streptococcus is the major offender.