In 23 children with untreated precursor B-cell acute lymphoblastic leukaemia (ALL), the daily growth rate of the malignant cell clone was calculated. Cell birth expanded the leukaemic cell clone an average 10-11% per day, programmed cell death or apoptosis reduced the leukaemic cell mass by some 4% per day. From these two variables a net increase in the size of the leukaemic cell population of 6.9+/-7.3% (range -1.2-27.3%) per day could be calculated. The daily growth rate correlated negatively with the logarithm of the duration of clinical symptoms before the diagnosis of ALL was established (r=-0.680; P=0.0004). A long history, especially in children with undefined bone pain and arthralgias, was associated with a very slow expansion of the neoplastic cell clone.