The nationwide Swedish Childhood Diabetes Registry, which ascertains 99% of recent-onset Type 1 (insulin-dependent) diabetic children (0-14 years) in Sweden, was linked with the Swedish Medical Birth Registry. A matched case-control study was carried out analysing about 20 perinatal variables concerning mother and child. A total of 2757 infants who became diabetic during the period 1978-1988 were analysed. For each case infant three control children were randomly selected from among all infants born in the same year and at the same delivery unit as the case infant. The following statistically significant risk factors were identified for Type 1 diabetes with an onset before 15 years of age: maternal diabetes (OR = 3.90), maternal age above 35 (OR = 1.36), maternal non-smoking (OR = 1.54), pre-eclamptic toxaemia (OR = 1.19), caesarian section (OR = 1.32), and maternal-child blood group incompatibility (OR = 1.61). When the analysis was restricted to Type 1 diabetes with an onset before the age of 5 years, most odds ratios were increased - for blood group incompatibility OR = 3.86 (95% confidence interval 1.54-9.65). Icterus without blood group incompatibility was not a significant risk factor. When each risk factor was analysed after standardization for all other risk factors, the odds ratios remained significantly increased. Scrutiny of medical records for cases and control children with a diagnosis of blood group incompatibility verified the diagnosis in close to 90% of children. The more severe cases needing phototherapy and/or blood transfusion were found to have a greater risk than milder cases.(ABSTRACT TRUNCATED AT 250 WORDS)