The photolytic decomposition of hydrochlorothiazide in water was investigated. Irradiation of the drug was performed in pure water and sewage treatment plant water with a solar simulator and by direct sunlight. Here we show that three photoproducts were obtained. Two of them were formed in yields much higher than 10%. An environmental risk assessment of hydrochlorothiazide should take into account also these photoproducts in a complete analytical and ecotoxicological evaluation.