The purposes of this study were to investigate the attentional characteristics of hyperactive children, the relationship of subjective and objective measures of these characteristics, and the effects of methylphenidate on these measures of attention. Forty-five hyperactive children, ages 6 to 10 years, were entered into an 18-week study of the effects of methylphenidate (Ritalin) on attention. Measures included rating scales completed by teachers and parents and a vigilance task. All measures reflected significant changes during the period of investigation. Attention and behavior were significantly improved under drug conditions and significantly worsened when methylphenidate was discontinued. However, only performance on the objective measure returned to predrug levels; final off-drug parent and teacher ratings remained improved over initial reports. Parent ratings of behavior, and specifically of children's ability to attend, were unrelated to equivalent teacher ratings. Teachers' ratings of attention correlated significantly with performance on the vigilance task, discriminated between on-drug and off-drug conditions, and discriminated between children who obtained normal or near normal predrug scores on the objective measure and those who performed poorly on this measure. Methylphenidate improved attentional performance for children who had poor predrug scores on the vigilance task, but did not produce a statistically significant change on the scores of children with normal predrug performance.