We have determined the prevalence of methylenetetrahydrofolate reductase (MTHFR) mutations C677T and A1298C in 71 children (< or = 15 years) with acute lymphoblastic leukaemia (ALL) and in 71 control subjects. Odds ratio (OR) for ALL linked to MTHFR C677T was 0.4 (95% CI 0.2-0.8); for heterozygotes it was 0.5 (95% CI 0.2-0.9) and for homozygotes it was 0.3 (95%CI 0.09-0.8). MTHFR A1298C yielded an overall OR for ALL of 1.3 (95% CI: 0.7-2.6); for heterozygotes it was 1.3 (95% CI: 0.7-7.6) and for homozygotes it was 2.8 (95% CI 0.5-15.6). In conclusion, MTHFR C677T was linked to a significant 2.4-fold decreased risk of developing childhood ALL, whereas MTHFR A1298C did not significantly affect the risk of ALL in our population.