Objective: Health-related quality of life (HRQL) is increasingly used to estimate needs for medical treatment, to evaluate its outcome and quality of care. The aim of this study was to compare the HRQL of several diagnostic groups before and after treatment with the HL-test (HL = IQL, Icelandic Quality of Life test) and to study its validity for measuring changes in quality of life. Material and methods: Patients on waiting lists for coronary catheterization, orthopedic or urologic operations, patients in psychiatric out-patient treatment and patients entering treatment for alcohol dependence were asked to fill in the HL-test, a total of 1195 patients. Three months after treatment they were retested. The results of tests were standarized with population norms available to make them directly comparable with those of the general population. Results: The response rate was 75% in each round. The HRQL of all patients was reduced in all aspects compared to that of the general population, that of the heart and urology patients less so than that of the orthopedic and psychiatric patients. Each group had a specific profile, especially marked for the orthopedic and psychiatric patients. Following treatment the HRQL or some aspects of it improved in all groups, especially for those which it had been most impaired. Conclusions: Studies of HRQL provide information useful for planning and delivery of health services. The HL-test is an instrument with good validity and reliability which is easy to use for such studies.