The parents of children with skin disease are often unsure how much topical therapy, particularly of corticosteroids, they should apply. The aims of this study were to devise simple guidelines on topical therapy for children, parents, doctors and nurses, and to check the accuracy of these guidelines in practice. The guidelines are based upon four principles: the adult fingertip unit (FTU); the 'rule of 9s'; standard height and weight charts for children; and standard nomograms for calculating body surface area. Twenty-four children (11 boys and 13 girls) aged 6 months to 9 years 4 months with atopic eczema were recruited and the number of FTUs required to treat different anatomical areas calculated in accordance with the proposed guidelines. Ointment was applied and the number of FTUs needed for each area was recorded. The amount used was then compared with that predicted. No child required a greater number of FTUs than that predicted, and the number of FTUs predicted for each anatomical region was accurate to within 1 FTU. The guidelines provide a useful indication of how much topical therapy is required for children, and advice sheets have been prepared for children of different ages.