The purpose of the study was to assess the prevalence of and factors associated with insomnia among enrollees of managed care organizations (MCOs). A survey was distributed either by mail or during a clinic visit to 7,500 enrollees of five MCOs in the United States. The survey included a sleep questionnaire, demographic questions, and questions about medical encounters and prescription drug use. Three levels of insomnia (none; level I--difficulty initiating or maintaining sleep; level II--insomnia with daytime dysfunction) were defined from the responses. Comorbidities were determined by proxy from prescription drug use reported by respondents. A total of 3,447 survey responses were received, yielding a response rate of 46%. Level I and level II insomnia was reported by 13.5% and 32.5% of the respondents, respectively. Level II insomnia increased with decreasing education, income, and age and was more prevalent in women and non-Caucasians. Insomnia was significantly correlated with all daytime sleepiness and most nighttime disturbances factors. Fifty-two percent of all respondents reported at least one comorbid condition. Respondents with multiple comorbidities reported level II insomnia more frequently than those with no comorbidities. Only 0.9% of clinic visitors were seeing a physician specifically for sleep problems. Of those with level I and level II insomnia, only 5.5% and 11.6%, respectively, were taking prescription medications specifically for sleep problems; 11.2% and 21.4%, respectively, were taking over-the-counter medications for sleep. Insomnia occurs in MCO enrollees at rates comparable to those found in the general population. However, few patients with insomnia are actually being treated for their condition. Proper evaluation, diagnosis, and treatment of insomnia are warranted.