This study investigated the effects of an Internet-based intervention for insomnia. Participants who met criteria for insomnia (N = 109) were randomly assigned to either a cognitive-behavioral self-help treatment or a waiting list control condition. The 5-week intervention mainly consisted of sleep restriction, stimulus control, and cognitive restructuring. Sleep diary data were collected for 2 weeks at baseline and at posttreatment. The dropout rate was 24% (n = 28). Results showed statistically significant improvements in the treatment group on many outcome measures, including total sleep time, total wake time in bed, and sleep efficiency. However, improvements were also found in the control group. Overall, between-groups effect sizes were low, with the exception of the Beliefs and Attitudes About Sleep Scale (Cohen's d = .81).