Most patients with fibromyalgia (FM) have a significant problem of insomnia, which is usually considered as a simple consequence of pain. Dysfunctional beliefs about sleep are one of the main maintenance factors of chronic insomnia. However no studies have analysed the possible contribution of these beliefs to the sleep problems observed in FM. In the present study, 90 women with FM and 70 healthy women completed several self-report measures about sleep, pain, mood state, and daily functioning. Both groups differed significantly in all the variables. In the clinical group, poor sleep quality correlates with greater pain, depression, anxiety and low level of functioning. In addition, patients with FM showed more dysfunctional beliefs about sleep, which correlate significantly with several measures. Pain intensity, depression and dysfunctional beliefs about the consequences of insomnia for health are significant predictors of poor sleep quality. The implications of these findings for current treatment of FM are discussed.