We evaluated the efficacy of cephalexin therapy in 104 patients with infectious maxillary sinusitis. Confirmation of diagnosis was made by x-ray film, antral puncture, and culture. Sixty-nine patients met all criteria for evaluation. The majority of patients received cephalexin, 1 gm/day in four divided doses for ten days, including the 48-hour period after the patient had become asymptomatic. Patients returned for observation one week and two weeks after initiating therapy. Satisfactory clinical results were obtained in 57 (83%). Results showed that cephalexin was effective in treating sinusitis and that adverse reactions were rare. Bacteriology results showed that many of the pathogens usually involved in infectious maxillary sinusitis were resistant to the recommended or frequently prescribed antibiotics. These same pathogens, however, did not show resistance to cephalexin.