This register study compared the rates and types of autoimmune disease in the parents of 111 patients (82 males, 29 females; mean age at diagnosis 5y 5mo [SD 2y 6mo]) with infantile autism (IA) with a matched control group of parents of 330 children from the general population. All parents were screened through the nationwide Danish National Hospital Register. We inquired about 35 autoimmune diseases during an observation period of 27 years. At follow-up the case and control mothers were identical in age (65y 7mo [SDs 9y 7mo and 9y respectively]). For case and control fathers the figures were 70 years 2 months (SD 10y 2mo) and 69 years 1 month (SD 10y 1mo) respectively. A similar proportion of case and control mothers had a diagnosis of any autoimmune disease: 10.8% versus 9.1%. For case fathers the proportion was 8.6% versus 4.6%. Two autoimmune conditions were associated with IA: ulcerative colitis in mothers (p=0.05) and type 1 diabetes in fathers (p=0.02). Additional studies are required to determine whether there is a true association between a parental history of autoimmune disease and pervasive developmental disorder in their offspring.