Objective: Our purpose was to investigate the efficacy and safety of 0.1% topical tacrolimus in erosive or ulcerative oral lichen planus. Methods: This was an open-label, noncomparative study conducted in an outpatient oral medicine unit in London, United Kingdom. The study covered an 8-week period with a 22-week follow-up after cessation of therapy. Nineteen patients, aged 28 to 87 years with biopsy-proven oral lichen planus refractory to, or dependent on, systemic immunosuppressive agents, were enrolled. Seventeen patients (89%) completed the study. Application of 0.1% tacrolimus was administered to all symptomatic oral mucosal lesions. Clinical review took place 1, 3, 5, 7, and 8 weeks after commencing therapy. Alleviation of symptoms was evaluated by using a visual analogue scale as well as the McGill Pain and Oral Health Impact profile questionnaires. The extent of the oral mucosal erosion or ulceration was directly measured by the same clinician at all visits. Safety assessments included monitoring of adverse events, complete blood cell count, renal and hepatic clinical chemistry, and tacrolimus blood concentrations. Results: Tacrolimus caused a statistically significant improvement in symptoms within 1 week of commencement of therapy. A mean decrease of 73.3% occurred in the area of ulceration over the 8-week study period. Local irritation (in 6 subjects, 35%) was the most commonly reported adverse effect. Laboratory values showed no significant changes with time. Therapeutic levels of tacrolimus were demonstrated in 8 subjects but were unrelated to the extent of oral mucosal involvement. Thirteen of 17 patients suffered a relapse of oral lichen planus within 2 to 15 weeks of cessation of tacrolimus therapy. Conclusion: Topical tacrolimus is effective therapy for erosive or ulcerative oral lichen planus. (J Am Acad Dermatol 2002;46:35-41.)