Fifty-four patients taking minocycline for acne or rosacea were assessed for adverse effects. Their mean duration of treatment was 17 months, and their average cumulative dose was 47 g. No symptoms attributable to the therapy were reported. Biochemistry and haematology profiles were normal. There was no evidence of an adverse effect on thyroid function. Skin pigmentation was detected in eight patients (14.8%). Five patients had diffuse facial pigmentation, and three patients had localized pigmentation at the site of a scar or injury. Diffuse pigmentation occurred only in patients who had been on treatment for 3 years or more; 50% of such patients were affected. Age and solar damage may also have been factors in this type of pigmentation. Localized pigmentation occurred at sites of previous tissue damage, and was not directly related to the duration of therapy. Patients who receive long-term minocycline therapy should be regularly monitored for the development of pigmentation.