During the 3 years 1984-86, 314 cases of multiple myeloma were diagnosed in the Health Care Region of Western Sweden. 180 of these cases were included in a clinical trial; 71 were notified to the trial but excluded; 49 cases were not reported to the trial; 14 were diagnosed post mortem. The crude incidence rate of myeloma was 6.3 cases per 100,000 inhabitants per year, corresponding to an age-adjusted (world standard population) incidence rate of 2.9 cases per 100,000 inhabitants per year. The excluded and the non-reported patients had a significantly shorter survival than those included in the clinical trial (median survival 22, 13 and 33 months, respectively). This was partly due to differences in age and proportion of actively treated patients between the groups, but the same tendency remained also after correction for these factors. Considering the included patients separately, the effect of tentative application of presumptive exclusion criteria corresponding to major prognostic factors was studied. Prolonged survival was seen when the upper age limit was lowered and when patients with renal failure or low performance status were excluded. The results illustrate the fact that for multiple myeloma the survival in a trial population is markedly influenced by active and passive exclusion of patient groups with unfavourable prognostic characteristics. When reporting results of clinical trials, discussion of the representativeness of the trial population for the total patient population is recommended in order to facilitate application of the trial results to medical practice and comparisons between trials.