OBJECTIVE. It has been hypothesized that sleep deprivation may manifest in children as behavioral symptoms rather than as tiredness, but only a few studies have investigated this hypothesis. The objective of our study was to evaluate whether short sleep is associated with behavioral symptoms of attention-deficit/hyperactivity disorder in 7- to 8-year-old children. METHODS. We performed a cross-sectional study of children born in 1998 in Helsinki, Finland. The participants included 280 (146 girls, 134 boys) children with a mean age of 8.1 years (SD: 0.3; range: 7.4–8.8). Sleep quality was measured by using actigraphs. The Sleep Disturbance Scale for Children and the Attention-Deficit/Hyperactivity Disorder Rating Scale IV were administered to parents. RESULTS. Children whose average sleep duration as measured by actigraphs was short (<10th percentile, ie, <7.7 hours) and had a higher hyperactivity/impulsivity score (9.7 vs 7.8 or 7.5) and a higher attention-deficit/hyperactivity disorder total score (17.3 vs 14.5 or 13.1) but a similar inattention score (7.6 vs 6.7 or 5.6) compared with children sleeping 7.7 to 9.4 hours or >9.4 hours. In multivariate statistical models, short sleep duration remained a statistically significant predictor of hyperactivity/impulsivity, and sleeping difficulties were associated with hyperactivity/impulsivity, inattention, and the total score. There were no significant interactions between short sleep and sleeping difficulties. CONCLUSIONS. Children's short sleep duration and sleeping difficulties increase the risk for behavioral symptoms of attention-deficit/hyperactivity disorder.