Healthcare costs are continuously increasing, and impose a strong responsibility on governments for an adequate allocation of resources among healthcare provisions and patients. The aims of the present study were to describe the healthcare costs of intellectual disability (ID) and other mental disorders in the context of the total costs of all other diseases, and to determinate the future need of healthcare resources, especially for ID and mental disorders. The present authors performed a top-down cost-of-illness study comprising all healthcare costs of the Netherlands in 1994. Data on healthcare use were obtained for all 22 healthcare sectors, and used to ascribe costs to disease groups, age and sex. Costs of mental disorders are by far the largest in the Dutch healthcare system. Some 25.8% of total disease-specific costs could be ascribed to mental disorders: psychiatric conditions, 10.6%; ID, 9.0%; and dementia, 6.2%. There are large differences between age and sex groups. The costs of ID and schizophrenia are higher among men, and the costs of dementia and depression are higher among women. The age pattern shows two peaks: the first occurs at 25-35 years of age (ID and psychiatric conditions); and the second at 75-85 years of age (dementia). Time trends between 1988 and 1994 show an average annual growth rate of 5.2% for total healthcare costs: psychiatric conditions, 4.8%; ID, 5.4%; and dementia, 9.4%. Demographic projections suggest a less-than-average cost increase for ID and psychiatric disorders (with annual growth rates of 0.2% and 0.4%, respectively) compared to the costs of dementia and total healthcare (with annual growth rates of 1.6% and 0.9%, respectively). Intellectual disability and mental disorders represent a large part of healthcare use in the Netherlands. The costs will inevitably increase because of the ageing of the population and increasing life expectancy among people with disabilities. Non-specific cost containment measures may endanger the quality of care for vulnerable people at younger and older ages.