The first study of photodynamic therapy in the human gastrointestinal tract using 5 aminolaevulinic acid (ALA) induced protoporphyrin IX as the photosensitising agent is described. Eighteen patients with colorectal, duodenal, and oesophageal tumours were studied. After 30-60 mg/kg of ALA given orally, biopsy specimens of tumour and adjacent normal mucosa were taken 1-72 hours later. These specimens were examined by quantitative fluorescence microscopy for assessment of sensitisation with protoporphyrin IX. Ten patients were given a second dose of ALA a few weeks later and their tumours were treated with red laser light (628 nm). With 30 mg/kg ALA, the highest fluorescence values were detected in the duodenum and oesophagus, and the lowest in the large bowel. Doubling the ALA dose in patients with colorectal tumours gave protoporphyrin IX fluorescence intensities similar to those in patients with upper gastrointestinal lesions and improved the tumour:normal mucosa protoporphyrin IX sensitisation ratio. The treated patients showed superficial mucosal necrosis in the areas exposed to laser light. Six patients had transient rises in serum aspartate aminotransferases, two mild skin photosensitivity reactions, and five mild nausea and vomiting. In conclusion, photodynamic therapy with systemically administered ALA may be a promising technique for the treatment of small tumours and areas of dysplasia such as in Barrett's oesophagus.