Seven regional studies on the population of long-term users of mental health care (i.e. more or a less continuous use over the past two years) in the Netherlands are compared. The prevalence of long-term mental health users in the Netherlands is estimated at about 3,0 per 1,000 of the population. The population density is a good predictor of regional differences in prevalence. A description is given of the sociodemographic and clinical characteristics of the population of long-term users and the content of mental health care received. The population is remarkably heterogeneous both in terms of psychiatric diagnosis and pattern of care. The validity of the research method and the definition of long-term use of mental health care are discussed. The studies were confined to actual mental health care consumers. Additional information of general practitioners gathered in three studies suggests that the total prevalence of the chronically mentally ill is at least 4,5 per 1,000 inhabitants.