SUMMARY From 1944 through 1953 at the University of Arkansas Medical Center, therapy of 24 patients with primary lung abscess was inadequate: six died, ten required surgery, and only nine (38 per cent) had a good result. In contrast, of 63 patients treated from 1954 through 1963, only two died, 14 had surgery, and 55 (87 per cent) had a good result. In this group, age significantly influenced response; seven of eight with poor responses were over 60 years old. An analysis of the effect of antibiotics revealed that penicillin was very effective. In 51 patients given penicillin only, one died and three had inadequate healing, while surgery was necessary in only seven (14 per cent). In contrast, of 12 patients receiving combined antibiotics, one died, three had inadequate healing, and four required surgery. These results indicate that penicillin alone is adequate antibiotic therapy for primary pyogenic abscesses.