Local application of 5-Fluorouracil was practised on 5 patients. In 3 patients with multiple premalignant oculo-cutaneous lesions, two of whom had also premalignant epibulbar lesions, local 5% 5-Fu cream and/or 1% 5-Fu eyedrops were applied. In 2 other patients who had only premalignant lesions on the cornea and conjunctiva, 1% 5-Fu eyedrops were used only. In animal experiments 1% 5-Fu was not found to cause any damage to normal corneal and conjunctival epithelium. The frequency of the applications and the duration of the treatment were dependent on the location and extent of the lesions. Except for easily treatable lesions, caused by the separation of the tumour epithelium, no complications of local 5-Fu treatment were seen. The visual acuity improved in all 4 patients in whom the cornea was affected. In these premalignant conditions the diagnosis and assessment of the therapy were based on exfoliative cytology and biopsies.