The British National Formulary and many reference textbooks recommend that drug dosages for children be calculated according to body surface area (BSA). Although many rules for drug dosage have been developed, based on age, weight and surface area, none has been accurate and simple enough for routine use. These rules are described, and one for clinical use: up to 30 kg, a child's drug dose may be (wt x 2)% of an adult dose; over 30 kg, (wt + 30)% of an adult dose. If this percentage of an "adult" dose of a drug is used, not only is the BSA curve followed more closely than with the conventional mg kg-1 regimen, but fewer major errors of prescription may be expected.