This paper describes the concept and realization of a nation-wide observational study which is part of the patient care evaluation programme on acupuncture as part of a model project of German statutory insurance bodies. The study aimed describing medical care with acupuncture to estimate the incidence of adverse reactions as well as the treatment effects of acupuncture in chronically ill patients with headache, low back pain or osteoarthritic pain. It was also intended to analyse relations between treatment effects and specialties of the treating acupuncturists. During the two-year period of report 9,918 acupuncturists took part in the study documenting 503,397 cases. A more comprehensive documentation is available for a sample of 10,366 patients with complete longitudinal data of 6,140 patients. The difficulties with data management and sampling technique are presented with reference to restricted capacities for monitoring. Furthermore the question of generalising of the results is discussed. There were differences in some respects (for example, frequencies of different indications) between the patient sample and the population of all patients included in the project. Patients also differed slightly with respect to the completeness of the longitudinal documentation. The results indicated that we cannot claim global representativeness for the sample-based findings, but there is no reason to assume a "positive" selection.