Boys with Asperger syndrome (n=20), attention-deficit-hyperactivity disorder (n=20), and reading and writing disorder (n=20) were followed up and retested on several neuropsychological measures 1 to 2 years after initial assessments. Wechsler Intelligence Scale for Children (WISC-III) Full Scale, Verbal, and Performance IQ scores remained stable for all diagnostic groups. Kaufman factors and 'fluid' and 'crystallized' abilities were also stable measures. Subtest stability over time, was slightly more variable. There was a tendency for the group with Asperger syndrome to deteriorate over time with respect to logical reasoning abilities. Measures of executive function/attention ('go-no-go' and 'conflict' tests) showed good test-retest stability in all diagnostic groups. This is the first study of its kind.