A placebo-controlled, double-blind study of immunotherapy with the mould species Cladosporium was performed in 22 adult asthmatics. The diagnosis of Cladosporium allergy was based on a combination of bronchial provocation test and daily symptom score in the Cladosporium season. An aqueous preparation of a potent, biologically standardized and purified extract was used in a clustered dose-increase regimen. The clinical efficacy was evaluated by a combination of symptoms (asthma score + peak flow) and consumption of antiasthmatic medication. The mean changes in symptoms and medication consumption over a 10–week registration period (peak Cladosporium season) in 1982 after 5–7 months of immunotherapy were compared with the corresponding 1981 pretreatment 10-week period A significant (P= 0.03) difference in terms of “improved”, “unchanged” and “deteriorated” patients in favour of Cladosporium treatment was found. Approximately 80% in the Cladosporium group showed improved/unchanged symptoms contrary to 30% of the placebo treated. Side effects were observed frequently but only in the Cladosporium-treated. About 70% experienced a large local reaction and 100% had episodes of asthma during dose-increase phase. Only a few severe systemic reactions occurred. Based on the clinical efficacy of the treatment we consider immunotherapy with Cladosporium feasible for highly specialized clinics.