84 patients with oral lichenoid lesions (OLL) were seen in the contact dermatitis clinic. All these patients had reticulate, lacy, plaque-like or erosive lichenoid changes adjacent to amalgam fillings. Patch testing to metallic mercury, 0.1% thimerosal, 1% ammoniated mercury, 0.1% mercuric chloride, and in some cases 0.05% phenylmercuric nitrate and amalgam discs was undertaken. 33 (39%) patients had positive patch test findings. 30/33 patch test positive patients had replacement of their amalgam fillings, with 28 (87%) patients experiencing improvement of symptoms and signs within 3 months. This confirms that mercury allergy is a factor in the pathogenesis of OLL in some cases. In cases where patch test negative patients improve with amalgam replacement, mercury may be acting as an irritant in the pathogenesis of OLL.