One of the most serious complications of chickenpox is varicella gangrenosa, a form of necrotizing fasciitis. A confounding factor is the frequency of more superficial skin infections during chickenpox, often caused by group A beta-hemolytic streptococcus. Because the progression from cellulitis to fasciitis is so insidious, severe tissue necrosis often has occurred before the correct diagnosis is made on the basis of clinical examination. Even today, limb amputation is occasionally required for treatment of gangrenous varicella. In this report, we demonstrate that magnetic resonance imaging can delineate the degree and depth of inflammation within an extremity and, thereby, indicate when the infection is progressing despite antibiotic therapy. Thus, the earlier use of magnetic resonance imaging may facilitate the diagnosis of necrotizing fasciitis.