Large patches of Bowen's disease (intraepidermal carcinoma in situ) can be difficult to treat by conventional methods. Photodynamic therapy (PDT) uses the combination of a photosensitizer, which preferentially accumulates in malignant cells, and photoactivation by visible light to kill the malignant cells. 5-aminolaevulinic acid (ALA) PDT uses excess exogenous ALA, which produces, via the haem synthesis pathway, a build up of the photosensitizer protoporphyrin IX. We describe the use of topical ALA PDT to treat three patients with three especially large patches of Bowen's disease. Following two treatments all three lesions achieved a complete clinical and histological response with a good cosmetic result. ALA PDT is a simple, effective and well tolerated treatment for large patches of Bowen's disease.