Eighteen patients with pyomyositis were seen at a large hospital in Hawaii. Most were healthy young men or boys, and eight had never traveled abroad. Fever, muscle pain and swelling, and leukocytosis were common, but only seven had erythema or fluctuance. A single large muscle was infected in 14, and two to three muscles were infected in the others. Pyomyositis mimicked cellulitis, muscle hematoma, thrombophlebitis, appendicitis, and neoplasm. Staphylococcus aureus was found in 13, and Streptococcus pyogenes was found in two. Bacteremia occurred in 29% (5/17). Drainage and antimicrobial agents were usually effective therapy, but one patient died and another had CNS damage. The pathogenesis is uncertain, but 12 had nonpenetrating muscle injury and 13 had pyoderma, suggesting that bacteria invade injured muscle via the bloodstream or lymphatic system.