We determined the proportion of survival variability explained by the usual prognostic factors in childhood acute lymphoblastic leukaemia (ALL) during a prognostic study of 1552 patients enrolled in three consecutive Fralle group protocols (Fralle 83, Fralle 87 and Fralle 89). The event-free survival rates at 5 years were 54.8% (SD 1.9), 43.1%) (SD 2.7) and 55.6% (SD 2.2), respectively. In the univariate analysis the following variables were predictive of poor outcome: male gender, elevated leucocytosis (> 50 x 10(9)/l), circulating blastosis. haemoglobin >12 g/dl, platelet count <100 x 10(9)/l, age under 1 year or over 9 years, enlarged mediastinum, nodes, spleen and liver, T phenotype, absence of CD10+ cells; testicular and meningeal involvement, poor response to induction therapy (CCSG M3), and LDH >400 U/l. Among the cytogenetic features, hyperdiploidy had a protective effect, whereas hypodiploidy, translocation and other structural abnormalities had a negative influence, particularly in cases of t(9;22) or t(4;11). Multivariate analysis summarized the prognostic information in terms of four variables: age, gender, leucocytosis and cytogenetic features. Missing data had little influence on the results. However, despite their significance in the multivariate analysis, these four variables each had very low predictive power (1.1% for gender, 2.0% for age, 3.5% for leucocytosis, and 1.6% for cytogenetic features). Thus, the most significant prognostic factors in childhood ALL each explain no more than 4% of the variability in prognosis. This may explain the disappointing practical value of these factors and underlines the need for prognostic tools in childhood ALL.