Background: Photodynamic therapy (PDT) has unique properties which make it suitable for the local treatment of superficial epithelial disorders; it has been suggested as a useful treatment for carcinoma in situ of the vulva. Objectives: To evaluate the effect of the systemic photosensitizing agent metatetrahydroxyphenyl-chlorin(mTHPC or temoporfin; Foscan., Biolitec, Edinburgh, U.K.) in vulval intraepithelial neoplasia type III (VIN III). Methods: PDT using mTHPC was performed in six patients with VIN III. A dose of 0.1 mg kg-1 body weight mTHPC was injected intravenously and the area of VIN irradiated 96 h later with 652-nm light from a diode laser. Patients were reviewed 1 week, 6 months and 2 years following treatment. Results: Patients experienced only minimal pain from the initial treatment but two patients subsequently developed severe pain at the treated site for up to 2 weeks following PDT. All patients developed oedema and slough formation at the treated site and one patient developed cellulitis. At 6 months two patients had developed small recurrences of VIN at the original site and one patient had an area of VIN at a new site. These were treated either with further PDT or with a small excision. At 2 years there was no recurrence of VIN at the original site in all patients reviewed. Conclusions: This small case series demonstrates that mTHPC-PDT is a useful initial treatment for VIN III. It is relatively selective, shows good cosmesis and conserves form and function. This is a major advantage over surgery. Repeat treatments are also possible, which is important in a condition such as VIN, which tends to be multifocal. Systemic mTHPC-PDT appears to have an advantage over topical 5-aminolaevulinic acid-PDT as the photosensitizer is distributedwidely in areas of disease and consequently identifies foci which may not be apparent clinically but become evident when illuminated. Objective: To evaluate the effect of the systemic photosensitizing agent Methods: PDT using mTHPC was performed in six patients with VIN III. A dose of 0.1 mg kg -1 (mTHPC or temoporfin; Foscan., Biolitec, Edinburgh, UK) in vulval intraepithelial neoplasia type III Body weight mTHPC was injected intravenously and the area of ​​VIN irradiated 96 h later with 652-nm light from a diode laser. Patients were reviewed for 1 week, 6 months and 2 years following treatment. Results: Patients experienced only minimal pain from the initial treatment but two patients subsequently developed severe pain at the treated site for up to 2 weeks following PDT. All patients developed oedema and slough formation at the treated site and one patient d Atveloped cellulitis. At 6 months two patients had developed small recurrences of VIN at the original site and one patient had an area of ​​VIN at a new site. These were treated either with further PDT or with a small excision. At 2 years there was no recurrence of VIN at the original site in all patients reviewed. Conclusions: This small case series demonstrates that mTHPC-PDT is a useful initial treatment for VIN III. It is relatively selective, shows good cosmesis and conserves form and function. This is a major advantage over surgery. Repeat treatments are also possible, which is important in a condition such as VIN, which tends to be multifocal. Systemic mTHPC-PDT appears to have an advantage over topical 5-aminolaevulinic acid-PDT as the photosensitizer is distributedwidely in areas of disease and identifying only foci which may not be apparent clinically but become evident when illuminated