A double‐blind histamine placebo controlled immunotherapy trial was performed to investigate the clinical effect of a purified and standardized Cladosporium herbarum allergen preparation. Thirty children with a clinical history suggesting mould‐induced asthma and/or rhinoconjuctivitis were included. The diagnosis was confirmed by positive skin prick test and Phadebas RAST® as well as positive bronchial and/or conjunctival provocation test to Cladosporium herbarum. Immunotherapy was given for 10 months in a double‐blind manner to randomized groups with either Pharmalgen®/Cladosporium herbarum preparation or histamine placebo. Allergic side effects to injections were common, especially during the peak of the mould season (July‐September in Scandinavia). In the active group, 13/16 patients experienced general reactions during the first 10 months of treatment. After 6 months of treatment, eye, nose and bronchial symptom scores and peak expiratory flow rates were similar for the groups, maybe because most of the children were also sensitive to many other allergens, including Alternaria alternala. However, medication scores were significantly lower in the treated group (P < 0.01). Bronchial (P < 0.01) and conjunctival sensitivity (P= 0.01) were significantly reduced in the Cladosporium‐treated group but not in the placebo group after 10 mouths of treatment. This is the first double‐blind clinical trial showing the clinical efficacy of immunotherapy in children with mould‐induced asthma.