In its usual form, disodium cromoglycate (DSG) contains lactose as a carrier (DSGL +). It has been suggested that lactose may effect irritant receptor sites, thus causing a degree of bronchoconstriction or less blocking of exercise-induced bronchospasm (EIB). This study was designed to assess and compare the ability of three different forms of DSG to block EIB. Eighteen asthmatic children and adolescents were given either DSGL+ by Spinhaler, DSG without lactose (DSGL-) by Spinhaler, nebulized (and also lactose-free) DSG solution (DSGN), or placebo in a randomized double-blind fashion on four occasions within a 10-day period. Pulmonary function tests were performed before and after medication and again after a standardized treadmill exercise test. After the exercise that followed treatment with each form of DSG the decreases in peak expiratory flow rate (PEFR) and forced expiratory volume in one second (FEV1) were significantly less than that seen after treatment with placebo. There was no significant difference between any of the three preparations of DSG. A complete block of EIB (decreases in PEFR and FEV1 of less than 12.5% and 10%, respectively) was seen in ony 11% of studies using all forms of DSG however, DSG was considered to be 'clinically effective' in 59% of studies.