Recent health legislation in the Netherlands makes specific provisions for patients who are not (fully) capable of acting reasonably in their own interests. If this legislation is actually to improve the legal position of incompetent people, it must be clear to whom these provisions apply. The description in modern legislation is that the person 'is regarded as not being able to make a reasonable assessment of his interests in the issue in question'. The law does not provide criteria according to which competence can be assessed in practice. In the present study, data on the competence assessment of 133 clients were collected by means of a poll held among those people who decide on competence in practice and a total of 485 assessors were polled. Client files were also examined with the intention of gaining insight into the carers' motivation to involve or exclude clients from the decision-making process. Finally, interviews were conducted with clients who have a mild or moderate intellectual disability. Large differences appeared to exist between the various assessors in the assessment of the general competence of clients. A general assessment of a client's competence is not sufficient in daily practice; therefore, partial competence was also assessed. Opinions about the partial competence of clients varied considerably among assessors. A ranking was compiled of discriminating characteristics in the qualification of competence or incompetence.