A new approach to photodynamic therapy (PDT) has recently been developed using the nature porphyrin precursor 5-aminolaevulinic acid (ALA) which is an intermediate in the haem biosynthetic pathway. It has been shown that using either topical or systemic administration of ALA, a transient accumulation of photoactive endogenous porphyrin species occurs which can then be exploited for photodynamic therapy. Until recently, only topically applied ALA has been used therapeutically which has restricted its use mainly to skin lesion treatment, but with systemic administration the potential clinical applications are much wider. In this article recent pre-clinical and clinical results obtained using systemically administered ALA are reviewed with the emphasis on treatment of gastrointestinal and oral cavity tumours. The use and possible limitations of ALA-induced endogenous porphyrin fluorescence as a diagnostic technique are also discussed.