Histories of seventy-four single-born white children with Legg-Perthes disease in Massachusetts in 1964 were compared with histories of a matched series of children without the disease selected from birth certificates. Sources of information included birth certificates, records of the hospitals of birth, and interviews with parents.

No differences between the case and comparison children were found in parental characteristics, frequency of complications of pregnancy and delivery, or frequency of injuries or illnesses in childhood. The case children differed significantly from the comparison children in having a lower mean birth weight. The proportion of smokers among the mothers of case children was higher than among the mothers of the comparison children.

A second series of 168 cases of Legg-Perthes disease diagnosed in other years, together with a series of matched comparison births, was used to supplement the data on birth weight. Some unexplained inconsistencies in the comparison groups in the two series were found. However, in the data from the two series combined, there was a highly significant excess of children of low birth weight among those with Legg-Perthes disease. The data indicate that male infants of birth weight under five and a half pounds are five times as liable to the disease as male infants weighing eight and a half pounds or more at birth.