The role of breast-feeding in protecting against childhood acute leukaemia and lymphomas is uncertain. We investigated this issue in a case-control study comprising 117 patients, aged 2-14 years, with acute lymphocytic leukaemia (ALL), Hodgkin's (HL) and non-Hodgkin's lymphoma (NHL), as well as 117 controls matched for age, sex and ethnicity. Information was collected via a telephone interview of the mothers. The median duration of breast-feeding among patients was significantly shorter than among controls, 7 (range 0-23) and 10 (range 0-20) months, respectively (P<0.0001). Breast-feeding of 0-6 months' duration, when compared with feeding of longer than 6 months, was associated with increased odds ratios (OR) for ALL (OR=2.47, 95% confidence interval (CI) 1.17-5.25), HL (OR=3.75, 95% CI 0.80-18.69), NHL (OR=4.06, 95% CI 0.82-22.59), and overall (OR=2.79, 95% CI 1.54-5.05). In the patient group, there were a significantly higher number of children and people per family, and patients were of a higher birth order than controls. In multivariate analysis, breast-feeding duration continues to be an independent predictor of lymphoid malignancies (P=0.015). In conclusion, breast-feeding lasting longer than 6 months may protect against childhood acute leukaemia and lymphomas.