Clinical studies of patients with melasma have shown that topical 20 percent azelaic acid is superior to 2 percent hydroquinone and as effective as 4 percent hydroquinone, without the latter's undesirable side effects. Tretinoin appears to enhance this effect of azelaic acid. Azelaic acid with tretinoin caused more skin lightening after three months than azelaic acid alone, and a higher proportion of excellent responders at the end of treatment. The effect of azelaic acid can be attributed to its ability to inhibit the energy production and/or DNA synthesis of hyperactive melanocytes, and partially to its antityrosinase activity. This may also account for the beneficial effect on postinflammatory hyperpigmentation. Destruction of malignant melanocytes by a combination of the same activities, enhanced by the greater permeability of tumoral cells to azelaic acid, may account for the clinical effects of azelaic acid observed in lentigo maligna and individual lesions of primary melanoma.