This paper reports the prognostic significance of clinical and laboratory features recorded at presentation in 485 patients entered into the Medical Research Council's 3rd therapeutic trial in myelomatosis between July 1975 and August 1978. The data were complete up to 1 January 1980, with a median follow-up time of 36 months. The 3 major determinants of prognosis were the blood urea concentration (BUC), the haemoglobin concentration ([Hb]), and the clinical performance status. Three prognostic groups based on these determinants were specified. The groups contained 22%, 56% and 22% of the patients and gave 2-year survival probabilities of 76%, 50% and 9% respectively. Patients in the good-prognosis group had a BUC less than or equal to 8 mM. [Hb] greater than or equal to 100 g/l, and no or minimal symptoms. Those in the poor-prognosis group had either [Hb] less than or equal to 75 g/l or a BUC greater than 10 mM and restricted clinical activity. Patients who had combinations of the 3 determinant features which excluded them from these 2 groups were classified into an intermediate prognosis group.