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Background: Tinea unguium is a common problem seen in clinical practice. Considering the many differential diagnoses of dystrophic nails, it is important to make a definitive diagnosis of dermatophyte infection before the initiation of antifungal therapy. Potassium hydroxide (KOH) preparation and fungal culture, which are commonly used in the diagnosis of these infections, often yield false-negative results. Recent reports have suggested that nail plate biopsy (Bx) using periodic acid-Schiff (PAS) (Bx/PAS) stain may be a very sensitive technique for the diagnosis of tinea unguium. Objective: The purpose of this study was to evaluate the usefulness of PAS and Grocott's methenamine silver (GMS) staining of nail specimen in the diagnosis of tinea unguium as a standard method. Methods: We evaluated 75 nail specimens from suspected tinea unguium using KOH preparation, biopsy using periodic acid-Schiff stain, and Grocott's methenamine silver stain. Results: Of the 75 nails which were negative on potassium hydroxide mounting, 43 and 39 cases were tested positive respectively on periodic acid-Schiff stain and Grocott's methenamine silver stain. Conclusion: Bx/PAS and Bx/GMS are the sensitive methods for the diagnosis of tinea unguium. They are indicated if clinical suspicion of onychomycosis is high and KOH preparation shows no fungal elements.