The aim of this study was to determine whether hypothyroid children treated early as a result of diagnosis after neonatal screening progressed normally in school. The New England Congenital Hypothyroidism Collaborative studied 72 of its patients at the ages of 9 or 10 years after they had completed 3 years of schooling beyond kindergarten. Control subjects were 96 classmates of the same sex and age as the patients and 32 siblings of appropriate age for the testing. Intelligence quotients (IQs) were measured by the Wechsler Intelligence Scale for Children-Revised abbreviated to five items, and school achievement by the Peabody Individual Achievement Test. In addition, a series of neuropsychologic tests was administered to both patients and control subjects. Educational histories were obtained from the parents for 74 patients, 87 of their siblings, 96 classmates, and 96 siblings of the classmates. The IQ of the control subjects (mean +/- SEM) was 109 +/- 1.2 versus 106 +/- 1.4 for the patients. The mean overall achievement score was 109 +/- 0.93 for the control subjects and 108 +/- 1.3 for the patients. These differences are not statistically significant. The regression lines relating overall Peabody Individual Achievement Test scores or subtests thereof to IQ did not differ among the patients and the control groups. Within the groups the regression lines for IQ and the different subtests of school achievement were also identical. The percentages of children repeating a grade, needing extra tutoring, or in special classes were the same for patients and control groups. We conclude that children with hypothyroidism have no apparent specific impediments to learning unrelated to intelligence.