Photodynamic therapy (PDT) produces local tumor necrosis, on activation of a previously administered sensitizer with non-thermal light of an appropriate wavelength. It is attractive for treating tumors of the mouth as tissue healing is particularly good. We describe the use of the photosensitizing agent meta tetrahydroxyphenyl chlorin (mTHPC, Foscan®) for PDT of oral cancer, including patients with field cancerization. Nineteen patients with histologically confirmed oral cancer (8 with field change disease) and one with severe dysplasia, were sensitised with mTHPC intravenously. Activation was carried out 72–96 hr later with laser light at 652 nm using a range of light doses. The results were assessed clinically and histologically. Multiple biopsies were taken during the ulcerative stages to look at the effects of PDT and after healing to assess the overall treatment result. All single lesions up to stage T3 cleared after one PDT treatment (total of 6 patients). Three out of 6 T4 tumours were also cleared. Lesions in patients with field change disease did less well, only 9 of 14 T1 and T2s clearing, including 4 that required extra treatments with a higher light dose. Most healed very well, but tongue tethering was seen in 1 patient and another had necrosis in normal areas due to light scattering within the mouth. PDT using mTHPC is a promising new treatment for patients with oral cancer. Int. J. Cancer 73:25–32, 1997. © 1997 Wiley-Liss, Inc.