Most fetuses in developed countries are exposed in utero to diagnostic ultrasound examination. Many pregnant women express concern about whether the procedure harms the fetus. Since most routine ultrasound examinations are done at weeks 16-22, when the fetal brain is developing rapidly, effects on neuronal migration are possible. We have sought an association between routine ultrasonography in utero and reading and writing skills among children in primary school. At the age of 8 or 9 years, children of women who had taken part in two randomised, controlled trials of routine ultrasonography during pregnancy were followed-up. The women had attended the clinics of 60 general practitioners in central Norway during 1979-81. The analysis of outcome was by intention to treat: 92% of the "screened" group had been exposed to ultrasound screening at weeks 16-22, and 95% of controls had not been so exposed, but there was some overlap. 2428 singletons were eligible for follow-up, and the school performance of 2011 children (83%) was assessed by their teachers on a scale of 1-7; the teachers were unaware of ultrasound exposure status. A subgroup of 603 children underwent specific tests for dyslexia. There were no statistically significant differences between children screened with ultrasound and controls in the teacher-reported school performance (scores for reading, spelling, arithmetic, or overall performance). Results from the dyslexia test sample showed no differences between screened children and controls in reading, spelling, and intelligence scores, or in discrepancy scores between intelligence and reading or spelling. The test results classified 21 of the 309 screened children (7% [95% confidence interval 3-10%]) and 26 of the 294 controls (9% [4-12%]) as dyslexic. The risk of having poor skills in reading and writing was no greater for children whose mothers had been offered routine ultrasonography than for those whose mothers had not been offered the procedure.