Inappropriate diagnosis of necrotizing arachnidism. Watch out Miss Muffet--but don't get paranoid.

A 28-year-old man presented with painful progressive necrosis of the skin of his left leg which was associated with general toxicity and a high fever. A working diagnosis of necrotizing arachnidism was made, but no improvement followed two weeks of therapy with analgesics, antibiotics and antihistamines. His symptoms responded rapidly to prednisolone therapy, and a diagnosis of pyoderma gangrenosum was confirmed by biopsy. This subsequently proved to be associated with early acute myelomonocytic leukaemia.