Abstract: Onyxis or ingrowing of the nails and especially onychocryptosis, unguis incarnatus, or ingrowing of the toenails is treated by applying to the affected nail and adjacent cuticle and skin area a topical solution comprising, as the active ingredient, ferric subsulfate solution, and allowing the solution to remain in contact with the nail and the surrounding skin area, by repeating this application night and morning for several days, until the unguis incarnatus is significantly alleviated.