A double-blind controlled trial of hyposensitization with tyrosine-adsorbed Dermatophagoides pteronyssinus vaccine in 24 children with atopic eczema and immediate hypersensitivity to D. pteronyssinus failed to demonstrate superiority over placebo after a standard 8 month course of treatment. In a second phase, children initially administered active treatment were randomly allocated to continue with active treatment or switched to placebo for a further 6 months. The clinical scores suggest that prolonged hyposensitization may be more effective than placebo but the numbers were too small to permit confident conclusions. A dramatic placebo effect may have served to conceal any additional therapeutic effect from active treatment.