Two cases of black discoloration of the thyroid are reported, each in a young woman treated for acne; the first with minocycline alone, the second with tetracycline and minocycline. Light microscopy showed numerous brown granules in the cytoplasm of the follicular epithelium, with large brown deposits in the colloid, including, in Case 2, numerous crystalline forms. The association between drug therapy and pigment accumulation (in conjunction with animal experiments) was taken to indicate some form of causal relationship, and minocycline residues were anticipated. Histochemical studies indicated that the granules were lipofuscin and electron microscopy showed an increase in phagolysosomes but no novel structures. There was no evidence for persisting minocycline, or its derivatives, although these could not be excluded. The black thyroid appears to be a disorder at lysosome/substrate level, induced by drug action, particularly by minocycline, and manifested by an accumulation of heterogeneous intracellular material with the histochemical and electron microscopic appearances of lipofuscin.