Sleep difficulty is a prominent concern of cancer patients, yet there has been no large study of the prevalence and nature of sleep disturbance in cancer patients. This cross-sectional survey study examined: (a) the prevalence of reported sleep problems in patients attending six clinics at a regional cancer centre; (b) sleep problem prevalence in relation to cancer treatment; and (c) the nature of reported insomnia (type, duration, and associated factors). For three months, all patients attending clinics for breast, gastrointestinal, genitourinary, gynecologic, lung, and non-melanoma skin cancers were offered a brief sleep questionnaire. Response rate was 87%; the final sample size was 982. Mean age of respondents was 64.9 years (SD 12.5). The most prevalent problems were excessive fatigue (44% of patients), leg restlessness (41%). insomnia (31%), and excessive sleepiness (28%). Chi square tests showed significant variation among clinics in the prevalence of most sleep problems. The lung clinic had the highest or second-highest prevalence of problems. The breast clinic had a high prevalence of insomnia and fatigue. Recent cancer treatment was associated with excessive fatigue and hypersomnolence. Insomnia commonly involved multiple awakenings (76% of cases) and duration > or = 6 months (75% of cases). In 48% of cases, insomnia onset was reported to occur around the time of cancer diagnosis (falling within the period 6 months pre-diagnosis to 18 months post-diagnosis). The most frequently identified contributors to insomnia were thoughts, concerns, and pain/discomfort. In a multivariate logistic regression analysis, variables associated with increased odds of insomnia were fatigue, age (inverse relationship), leg restlessness, sedative/hypnotic use, low or variable mood, dreams, concerns, and recent cancer surgery. This study provides new information about sleep-related phenomena in cancer patients, information which will be useful in planning supportive care services for cancer patients.