In order to investigate the possible causal relationships between hyperactivity and educational underachievement that might account for their frequent co-occurrence, four groups of boys, defined by the presence or absence of hyperactivity and specific reading retardation, were identified in an epidemiological study of 7 8-year-old children. They were examined in detail by means of parental interviews and psychological tests and reassessed 9 years later at the age of 16-18 years on a similar range of measures. The findings provided little support for the idea that persistent reading disabilities either lead to the development of hyperactivity de novo or increased the likelihood that hyperactivity, when present, would persist. Similarly, although features of hyperactivity persisted to follow-up, there was little evidence that they either lead to the development of reading disabilities or increased the likelihood that reading disabilities, when present, would persist. Socioeconomic adversity and a history of speech therapy were more common in the group with both hyperactivity and reading disability, but the strength of these associations made it unlikely that these factors could account for the frequent co-occurrence of the two conditions.