OBJECTIVE - To compare the neuropsychological profiles of children with IDDM with a community control group at two time points: 3 months after disease onset and 2 years after the baseline assessment. RESEARCH DESIGN AND METHODS - A total of 123 children (age 3-14 years) with recent IDDM onset were compared with 129 community control subjects, stratified for age and sex, on standardized measures of general intelligence, attention, speed of processing, memory, learning, executive skills, and behavioral adjustment soon after diagnosis and 2 years later. Exclusion criteria were premorbid evidence of central nervous system disease or trauma, or English not spoken in the home. RESULTS - There were no differences between children with IDDM and control subjects on any measure at the initial assessment 3 months after disease onset. Two years later, children with IDDM tended to show a less positive change, relative to control subjects. In their standardized scores on measures of general intelligence, and significantly so on the vocabulary (P < 0.01) and block design (P < 0.05) subtests. Multivariate group differences were also apparent on speed of processing (P < 0.05) and learning (P < 0.01) subtests, reflecting smaller developmental gains in the children with IDDM when compared with control subjects. CONCLUSIONS - The findings are consistent with previous reports, suggesting that IDDM is associated with an increased risk of mild neuropsychological dysfunction. The skills most affected in this cohort were information processing speed, acquisition of new knowledge, and conceptual reasoning abilities. Clinicians and educators should be made aware of the risk of specific neuropsychological deficits in children with IDDM.