Bacterial cellulitis is a common problem, etiologic diagnosis often unrewarding and opinions differ on empiric therapy. The purpose of this study was to determine the major microbiologic causes of bacterial cellulitis in a walk-in Emergency Room setting. 94 cases in 89 patients with clinical signs of cellulitis were studied. The infection was closed in 74 cases and associated with an open skin lesion in 22. The infection site was aspirated with a suction air-buffer technique employing a Cameco handle for easier handling and stabilization of the aspiration needle. After exclusion of contaminated samples, positive cultures were obtained from 30 cases (31.9%). Cultures were positive in 30.6% of open lesions and in 36.4% of closed ones. Staphylococcus aureus was the most common organism, present in 11 cases, S. epidermidis in 8 and group A beta-hemolytic streptococci in 5. All S. aureus strains were methicillin-sensitive and only 1 was sensitive to penicillin. The most common site of infection was the lower extremity (59%). According to these data the optimal initial therapy for bacterial cellulitis in adults should be with drugs active against both staphylococci and streptococci.