We reviewed longitudinal nail biopsies performed at Henry Ford Hospital on patients with acquired nail dystrophies to see whether the procedure did, indeed, provide useful diagnostic information and to see which microscopic features were most helpful in histopathologic diagnosis. Clinical diagnoses included psoriasis, lichen planus, Darier's disease, isolated longitudinal defects, and diffuse thickening. We found that clinical diagnosis could be supported by histopathologic findings in eight of twenty cases of acquired nail dystrophy. Specifically, we were able to make a diagnosis of psoriasis in four cases, lichen planus in three cases, and Darier's disease in one case. The other twelve cases showed nonspecific eczematous changes. We conclude that the longitudinal nail biopsy may be a useful diagnostic tool in certain cases of acquired nail dystrophy.