A case of proximal subungual onychomycosis due to Microsporum canis in a 36-year-old woman is presented. The onychomycosis involved the left thumb and the little fingernails, with thinning of the nail plate and crumbling of the nail plate surface. A milky-white discoloration of the proximal portion of the left thumbnail was also evident. A 2-mm longitudinal nail biopsy showed a large number of fungal elements in the whole length of the nail plate. Fungal hyphae were more numerous in the ventral nail plate and produced detachment of the superficial nail plate. The nail bed was not invaded by fungal elements and was devoid of inflammatory changes. Proximal subungual onychomycosis is uncommon in immunocompetent individuals but has frequently been described in patients with AIDS. In our patient, in whom the proximal subungual onychomycosis was due to M. canis, there were no clinical or biochemical signs of immunodeficiency. Oral treatment with terbinafine, 250 mg/daily for 2 months, produced clinical and mycological cure.