For cellulitis that does not respond to conventional antimicrobial treatment, clinicians should consider, among other explanations, several noninfectious disorders that might masquerade as infectious cellulitis. Diseases that commonly masquerade as this condition include thrombophlebitis, contact dermatitis, insect stings, drug reactions, eosinophilic cellulitis (the Wells syndrome), gouty arthritis, carcinoma erysipelatoides, familial Mediterranean fever, and foreign-body reactions. Diseases that uncommonly masquerade as infectious cellulitis include urticaria, lymphedema, lupus erythematosus, sarcoidosis, lymphoma, leukemia, Paget disease, and panniculitis. Clinicians should do an initial diagnostic work-up directed by the findings from a detailed history and complete physical examination. In many cases, skin biopsy is the only tool that helps identify the correct diagnosis. Special tests may also be needed.