Some patients develop phototoxic or photoallergic reactions to sunlight, either as a result of genetic predisposition or metabolic disease, or after exposure to specific drugs or chemicals. The advice on avoiding exposure to sunlight and protective measures given on chapter ‘Premalignant lesions and malignant tumours’ is relevant, except that the causative radiation may penetrate clouds and glass windows and be active throughout the year, even in those areas with seasonal climates.