Serial intelligence tests in 38 patients with childhood moyamoya disease were evaluated. A total of 98 tests were administered. The IQ scores were classified into three categories: tests performed between the time of onset of symptoms and 5 years after the onset of symptoms (n = 44), tests performed 5-10 years after the onset of symptoms (n=32), and tests performed more than 10 years after the onset of symptoms (n=22). When more than one test was performed during each period, the mean of the IQ scores was used. The IQ tests were administered two or more times to 10 patients in the onset-5-years category, and 5 of them exhibited lower IQ scores on later tests. The IQ scores were significantly lower in the 5-10 years category and in the more than 10 years category (76.8 +/- 23.1 and 73.9 +/- 31.1, respectively) than in the onset-5-years category (92.9 +/- 22.7). The IQ scores for the 5-10 years category and the more than 10 years category did not differ significantly. The IQ in pediatric moyamoya disease begins to decrease after the onset of symptoms, but the decline eventually stabilizes more than 10 years after the onset of symptoms.