The roles of age and sex and their relationship to other prognostic factors were studied in 117 chronic myeloid leukaemia (CML) and in 256 chronic lymphocytic leukaemia (CLL) patients. Survival in CML was not related either to age at diagnosis or to sex. In contrast, the CLL patients classified into four age strata (less than 50, 50-59, 60-69, greater than 70 years) had an expected median survival (EMS) of 142, 101, 85 and 33 months respectively (chi 2 for heterogeneity = 35.59, P less than 0.0005; chi 2 for trend = 25.09, P less than 0.0005). Prognostic power was independent of sex, Rai stages, total tumour mass score (TTM), TTM distribution pattern, anaemia, thrombocytopenia, serum immunoglobulins and response to therapy. The relative survival rate (the ratio of patient's EMS and EMS in age- and sex-matched general population) was 0.40 in CLL patients and 0.13 in CML patients. Relative survival was more reduced in older CLL patients than in younger ones (0.37 vs 0.47, respectively), whereas relative survival was less reduced in older CML patients than in younger ones (0.18 vs 0.12, respectively). The results show that the age is a significant independent prognostic factor in CLL but not in CML. The difference in the effects of age on prognosis in CLL and CML most probably reflects the fundamental differences in their respective pathogeneses.