OBJECTIVE: To determine the impact of Streptococcus pyogenes (group A streptococcus, GAS) associated cellulitis on hospital expenditure in the United States. METHODS: This study analyzed data from the Healthcare Cost and Utilization Project (2000) funded by the Agency for Healthcare Research and Quality. Hospitalizations due to cellulitis were identified using ICD-9 codes 681-682 as principal diagnoses. We assumed that 30-50% of cellulitis-related hospitalizations were caused by GAS. RESULTS: In 2000, there were approximately 110,000-183,000 hospitalizations due to GAS-associated cellulitis. Adults aged 18-64 years accounted for 56% of the hospitalizations, followed by the elderly (65 years and older, 36%) and children (<18 years, 8%). The most common comorbid conditions associated with cellulitis were hypertension (28%), diabetes mellitus (13%) and congestive heart failure (10%). The mean and median length of hospital stay for cellulitis were about five and four days, respectively. Total annual hospitalization charges for GAS-associated cellulitis were estimated at $1.2-2.0 billion in year 2002 US dollars. Average charge per hospital stay was about $10,500. CONCLUSIONS: GAS-associated cellulitis represents a significant economic burden in the US. Total annual hospitalization charges estimated for GAS-associated cellulitis are significantly higher than the recent estimate by the Institute of Medicine (Vaccines for the 21st Century, A Tool for Decisionmaking, 2000) for all GAS-associated hospitalizations (