Flow cytometry with acridine orange was used in 111 cases of childhood acute lymphoblastic leukaemia (ALL) to determine simultaneously DNA index, cell cycle distribution and cellular RNA content in bone marrow samples. The overall incidence of DNA aneuploidy in the population was 40%. Significantly higher incidence of DNA aneuploidy was present in the L2 (70.6%) as compared to the L1 (34.1%) group (P = 0.007), using the FAB classification. No difference in the frequency of normal and abnormal DNA stemlines was found between newly diagnosed and relapsed patients. The L2 group had significantly higher proliferation (S phase = 12.4%) than L1 (S phase = 5.8%) (P = 0.01), but RNA content was not significantly different. The aneuploid group had significantly more (P = 0.01) cases with L2 morphology (28.6%) compared to the diploid group (8%) and proliferation was higher in DNA aneuploid (S-phase = 9.5%) compared to DNA diploid (S-phase = 4.7%) leukaemias. Likewise, RNA content was significantly higher in aneuploid than in diploid ALL (P = 0.006). These correlations between morphology, cell kinetics and DNA index elucidate biological properties of leukaemic cells with potentially prognostic value.