Inhaled corticosteroids are considered to be effective and safe to treat children with asthma. These drugs, often used as maintenance treatment, can, however, influence the HPA-axis, which might be reflected by the serum and urine cortisol concentration. The aim of the present study was to investigate the efficacy and safety of fluticasone propionate (FP) 100 microg administered twice a day via a Diskhaler for 3 mo. FP was tested in a double-blind randomized placebo-controlled parallel trial in a group of 34 children with moderate asthma who did not use inhaled steroids for at least 4 wk prior to the study. At home, symptoms and peak flow recordings (PEFR) were noted in a diary. At each visit lung function was measured, and serum and urinary cortisol were determined. During treatment, wheezing decreased and PEFR values increased in the FP group. FEV1 and PC20-histamine increased and the reversibility decreased in the FP group. All changes were significant, with the exception of the change in nocturnal PEFR. Four weeks after cessation of FP all parameters returned to pretreatment values. Serum cortisol did not change significantly in either treatment group. The decrease in urinary cortisol in the FP group was significant only if it was compared with the increase in urinary cortisol in the placebo group. We conclude that FP 100 microg given twice a day is effective in children with moderate stable asthma. Suppression of the HPA-axis by FP 100 microg given twice daily, although not likely, cannot be ruled out by this study since the absence of a significant decrease in urinary cortisol in the FP group could be due to an insufficient number of patients. Additional studies are required to solve this problem.