Oral mucosal pigmentation is an infrequently reported side effect of minocycline. Two patients with minocycline deposition within teeth and bone, demonstrated by fluorescence microscopy, are described. Minocycline is the only tetracycline reported to cause discoloration of the oral mucosa. This may be the result of deposition of an insoluble degradation product of minocycline in the underlying bone. Pigmentation is not necessarily dose-dependent and may take months or years to resolve.