The efficacy and safety of minocycline were compared with that of amoxicillin in the treatment of 58 patients with acute bacterial sinusitis. The most frequently isolated pathogens were streptococci, staphylococci, and Haemophilus influenzae. After therapy for a mean time of 11 days, clinical cure or improvement and bacterial eradication were evident in 100% of the patients treated with minocycline and in 95% of the patients treated with amoxicillin. Roentgenographic results indicated clearing or improvement in 91% of the minocycline recipients and in 70% of those who received amoxicillin. These differences between treatments were not statistically significant. A low incidence of generally mild adverse clinical experiences occurred in both treatment groups. Thus, minocycline and amoxicillin were equally safe and effective in the treatment of these patients with acute bacterial sinusitis.