A three-month controlled study was performed to assess the cumulative incidence of oral Candida carriage and thrush in patients starting to take betamethasone valerate aerosol (800 microgram/day) for control of their asthma. Four of 41 patients on the corticosteroid aerosol developed thrush compared with none of 40 in the control group. However, the number of cumulative saliva culture positives for C. albicans rose by a similar amount (approximately 20%) in each group. A simple mouthwash procedure was shown to have no prophylactic benefit in the aerosol group. Oral candidiasis was not, however, clinically important.