The laboratory data for 17 patients with group A beta-hemolytic streptococcal necrotizing fasciitis (GAS NF) were compared with data for 145 patients hospitalized for cellulitis during the same period. Admission values of C-reactive protein and creatine kinase were higher for patients in the group with GAS NF than for patients in the group with cellulitis (P<.001), suggesting that standard laboratory tests may be useful for the early differential diagnosis of GAS NF and cellulitis.