We report two cases of intrathecal methotrexate overdose. A 3-y-old girl with acute lymphoblastic leukaemia and a 4-y-old boy with Burkitt's lymphoma were to receive an intrathecal injection of methotrexate after completion of intravenous methotrexate infusion. Instead of 12.5 mg, they both received a dose of 125 mg. Both children developed generalized convulsion 3 h after the overdose, but afterwards recovered completely. Intravenous folinic acid and dexamethasone rescue were employed, but no attempt was made to exchange the cerebrospinal fluid. In addition to the staff's failure to check the drug label carefully, the marked resemblance of the two dose preparations of methotrexate (50 mg/5 ml and 500 mg/5 ml) may have been contributory.