The chronic leukaemias include two distinct chronic neoplastic disease states, namely chronic myelogenous leukaemia (CML) and chronic lymphocytic leukaemia (CLL). The aim of this study was to assess the utility of leucocyte count, neutrophil percentage and absolute lymphocyte count from differential complete blood count analyses as indicators of the possible presence of CML and CLL. Blood counts from 102 patients with histopathologically confirmed CML and CLL were compared with counts for 858 cancer-free control subjects. Optimal cut-off values were identified by selecting values with the highest sensitivity-specificity combination for each blood count parameter for the two diseases. The results indicated that any individual with mature-appearing lymphocytes at a level > 6.65 x 10(9)/l in the peripheral blood should be examined further for CLL, and that any individual with a leucocyte count > 18.0 x 10(9)/l or a neutrophil proportion > 72.6% should be investigated for CML.