44 children under 2 years of age suffering from recurrent or persistent wheezy bronchitis, completed a double-blind crossover trial comparing nebulised sodium cromoglycate and matching placebo. Analysis showed that treatment response was age-related. Sodium cromoglycate proved significantly superior to placebo in reducing night cough, sleep disturbance, wheeze and activity limitation in the 24 patients aged 12 months and above (mean 17.3) on entry to the study. Whereas no significant differences were observed in the 20 children below 12 months of age (mean 8.3). These findings were confirmed by weekly clinical assessment. Both age groups spent fewer days in hospital during the active treatment period. Final subjective assessments showed that the older age group, parents favoured cromoglycate treatment, whereas in the younger age group, parents favoured placebo, although neither reached statistical significance. Both age groups showed marked placebo response to nebulised water.