The aim of this study was to compare bambuterol oral solution (10 mg) administered once daily in the evening with terbutaline oral solution (0.075 mg/kg body weight) administered three times daily in 2-5-year-old children and to compare bambuterol tablets (10 mg or 20 mg) administered once daily in the evening with terbutaline tablets (2.5 mg) administered three times daily in 6-12-year-old children with asthma. The study was of an open, randomized, parallel-group design, and lasted 1 year. The primary objective was to evaluate safety (pulse rate, blood pressure, adverse events, hematology, and clinical chemistry). Plasma terbutaline concentrations were also measured. Evaluation of efficacy (FEV(1)) was a secondary objective. A total of 141 patients (83 boys, 58 girls) were randomized and treated with the study drugs, i.e., 43 patients in the terbutaline group (30 on oral solution and 13 on tablets) and 98 patients in the bambuterol group (62 on oral solution and 36 on tablets). A total of 11 patients discontinued the study: 3 were on terbutaline, and 8 were on bambuterol. There were no clinically important differences between treatment groups regarding pulse rate, or systolic or diastolic blood pressure. There were no clinically important findings in the laboratory tests (hematology and clinical chemistry). Both terbutaline and bambuterol were well-tolerated, and the reported adverse events were mostly mild or moderate. Mean steady state plasma terbutaline concentrations at the visits ranged between 8.0-11.5 nmol/L in the bambuterol tablet group and between 10.6-15.2 nmol/L in the terbutaline tablet group. The corresponding values in children on oral solution were 10.3-11.3 nmol/L in the bambuterol group and 7.5-9.7 nmol/L in the terbutaline group. FEV(1) measured in the 6-12-year-old children increased by more than 0.2 L in both treatment groups during the year in the study. In conclusion, bambuterol tablets or oral solution once daily and terbutaline tablets or oral solution three times daily showed a comparable and favorable safety profile.