The initial diagnostic bone marrow specimens from 90 consecutive, untreated patients with chronic lymphocytic leukaemia were examined for the pattern of lymphocytic infiltration in relation to clinical stage (International Workshop System) and survival. Three non-diffuse (interstitial, nodular, mixed nodular-interstitial) and one diffuse pattern were recognized. Generally, the bone marrow patterns correlated well with clinical stage: a non-diffuse pattern prevailed in early, and a diffuse pattern in later stages. However, with a Cox analysis of the covariate effect of clinical stage and bone marrow pattern on survival, the bone marrow pattern was shown to have independent prognostic significance in the early stage A in which a diffuse pattern carried a fourfold increase in death rate as compared to a non-diffuse pattern. These high risk patients could not be identified by the Rai substages of the International Workshop System.