More than 100,000 species of fungi have been identified worldwide, but only a few dozen are of medical significance. Because of the opportunistic nature of these organisms and the prevalence of immunodeficient conditions, the number of medically important fungi is increasing. Identification of the causative organism is of value in the choice of correct therapy. The development of new antifungal drugs has made it increasingly important to be able to distinguish one fungus from another. This article reviews the laboratory diagnosis of the fungi generally classified as the superficial mycoses and the dermatophytes. © 1989, American Academy of Dermatology, Inc. All rights reserved.