Background Photodynamic therapy (PDT) using topical 5-aminolaevulinic acid (5-ALA) has been suggested as an effective and tissue-conserving method of treating carcinoma in situ of the vulva. Objectives To evaluate PDT in patients with vulval intraepithelial neoplasia type III (VIN III). Methods Topical PDT was performed in six patients with VIN III. Five of the six patients had persistent disease following treatment with other modalities including 5-fluorouracil cream, cryotherapy, carbon dioxide laser ablation and excision. Each patient was treated once with a fluence of 150 J cm−2 using a broad-band light source (580–740 nm) 4 h after topical application of 20% 5-ALA. Patients were reviewed clinically at 1 month and 6 months after treatment. Results All of the patients developed initial erythema of treated sites, three with subsequent erosions. All patients had clinically evident persistent VIN III at 1-month review. Five patients have subsequently undergone surgical treatment and one is regularly reviewed. Conclusions This small uncontrolled study indicates that, as currently administered, a single episode of topical PDT is not effective in the management of treatment-resistant VIN III.