A 58-year-old man presented with whitish patches on both great toenails for four weeks prior to visiting our hospital; the patches spread rapidly to other finger- and toe-nails. Prior to presentation, the patient had been diagnosed with idiopathic thrombocytopenic purpura two months ago and Kaposi's sarcoma three weeks ago. The patient was treated with human immunoglobulin for five days, and then received prednisolone 40 mg bid. Serology showed that the patient was negative for HIV and results of other laboratory tests were normal. The KOH slide preparation of the nail scraping showed long septated hyphae and numerous arthrospores. The fungus culture revealed whitish downy colonies on the front side and wine-red reverse pigmentation on Sabouraud's dextrose agar. Trichophyton rubrum was isolated on fungus culture and slide culture. The internal transcribed space (ITS) regions of ribosomal DNA of the cultured fungus were identical to Trichophyton rubrum. Proximal subungual onychomycosis (PSO) is the rarest form of onychomycosis. PSO initially presents as whitish patch(es) on the proximal side of the nail plate(s). Because PSO shows whitish to yellowish patches on the nail plate, the result of KOH examination of nail scrapings from the nail plate is almost always negative. Herein, we report on a case of multiple PSO in a patient with classic Kaposi sarcoma and suggest a method for easy KOH scraping on PSO.