Repeated methotrexate absorption studies were performed under standard conditions in 127 children receiving either oral or intramuscular methotrexate for acute lymphoblastic leukaemia. There was marked variability in peak concentration, area under the serum concentration curve and clearance both between patients and in repeated studies on the same patient. Although the intramuscular route produced higher serum concentrations and AUC than the oral route, variability within and between patients was considerable and was most marked at higher concentrations. Neither age or sex could account for variation in methotrexate absorption or clearance. Intramuscular methotrexate, although producing higher serum concentrations and AUC, does not reduce the variability observed with oral administration. Prediction of subsequent methotrexate concentrations from the knowledge of one absorption profile is not possible.