A double-blind, cross-over study was undertaken to compare inhalation of betamethasone valerate (BV, 800 microgram daily) with sodium cromoglycate (SCG, 80 mg daily) in twenty children requiring bronchodilators for perennial asthma. Each treatment period lasted 4 weeks but statistical comparisons were made only in respect of the last 14 days of each therapy. When the children were using BV they required not only less of the bronchodilator drugs but had fewer symptoms and higher daily peak expiratory flow rates when taking SCG. Statistically, all these differences were highly significant. For 2 weeks before the main trial each child was given a placebo aerosol (single-blind) to assess severity of asthma. In comparison with this period, SCG was associated with a significantly increased peak expiratory flow rate a lower symptom score by day but not by night, but their usage of bronchodilators followed a similar pattern. When the BV period was compared with the placebo period, patients had an even more significant rise in peak expiratory flow rate, less day and night symptoms, and took hardly any bronchodilators. The response to the two drugs did seem to depend upon which was given first. No monilial infections were found, nor any measurable defect in adrenal response from either treatment. Betamethasone valerate is considered to be superior to sodium cromoglycate as a treatment for childhood asthma insufficiently controlled on bronchodilators.