OBJECTIVE--To see whether small daily doses of prednisolone have any adverse effect on short term linear growth in children with mild asthma. DESIGN--Double blind, random order crossover trial of two dosages of prednisolone. During run in and washout periods patients were given placebo. All treatment periods were of two weeks' duration. SETTING--Outpatient clinic referrals in a secondary referral centre. PATIENTS--14 Children (10 boys) aged 7-11 years with normal growth velocity during the previous year, no signs of puberty, and no history of receiving systemic or topical steroids during the two months before the study. One child was excluded because his pulmonary function deteriorated and another was withdrawn because of varicella. INTERVENTIONS--2.5 and 5.0 mg prednisolone daily given in divided dosage in the morning and evening. MAIN OUTCOME MEASURE--Growth of the lower leg as measured twice a week by knemometry. RESULTS--A significant reduction in mean growth velocity of the lower leg occurred in both prednisolone treatment periods. The mean difference between the run in period and the treatment period with 2.5 mg prednisolone daily was 0.63 mm/week (95% confidence interval 0.47 to 0.80 mm/week) and between the run in period and the treatment period with 5.0 mg prednisolone daily 0.57 mm/week (0.38 to 0.77 mm/week). CONCLUSION--Small daily doses of prednisolone suppress short term linear growth in children with mild asthma. The clinical relevance of this finding needs further study.