Aims: To investigate cognitive abilities and adaptive skills in children and adolescents with myotonic dystrophy type 1 (DM1) and correlate the findings to the cytosine-thymine-guanine (CTG) repeat expansion size. Method: Cognitive level was assessed in 55 children and adolescents with DM1 (31 males, 24 females; mean age 12y 1mo, SD 5y 1mo; range 2y 7mo-21y 5mo) divided into the following categories: severe congenital DM1 (n = 19), mild congenital DM1 (n = 18), and childhood DM1 (n = 18). The Griffiths Mental Developmental Scale, the Wechsler Scales, and the Vineland Adaptive Behavior Scales (VABS) for adaptive skills were used for this purpose. Results: Learning disability was found in 95% of the severe congenital group, 83% of the mild congenital group, and 89% of the childhood DM1 group. The more severe the form of DM1, the lower the full-scale IQ (FSIQ; r[subscript s] = 0.28, p = 0.044). The individuals with severe congenital and childhood DM1 had a significantly higher verbal IQ than performance IQ (severe congenital: mean difference 5.7, SD 5.7, p = 0.008; childhood DM1: mean difference 9.8, SD 18.0, p = 0.038). CTG repeat expansion correlated negatively with FSIQ (r[subscript s] = -0.63, p less than 0.006). Almost all participants showed poor results on the VABS. There was a positive relationship between cognitive level and adaptive skills in the mild congenital (r[subscript s] = 0.95, p less than 0.01) and childhood DM1 groups (r[subscript s] = 0.92, p less than 0.01). Interpretation: Children and adolescents with DM1 exhibit significant cognitive and adaptive problems.