Dicarboxylic acids from C8 to C14 are competitive inhibitors of tyrosinase in vitro, and here, the effect of a cream containing 15% azelaic acid (C9) on 3 cases of lentigo maligna is described. The lesions were treated for 90 days, with remarkable clinical and histological effect, maintained for up to 2 yr after cessation of treatment. Progress during treatment of one case was additionally monitored by electron microscopy, which revealed progressive elimination of abnormal melanocytes both basally and suprabasally, and their replacement by essentially normal cells engaged in normal melanogenesis. There was also progressive diminution in the general disorganization of the epidermis, and disappearance of lymphocyte response. It is concluded that dicarboxylic acids have a direct inhibitory and cytotoxic effect on abnormally active or structurally disordered melanocytes in lentigo maligna, but further investigations are required to establish their precise mode of action. Similar application of dicarboxylic acids to normal skin affects only a small proportion of melanocytes, suggesting that some phasic factor, or individual states of activity, may be concerned in their susceptibility.