In a pilot study 28 elderly patients suffering from either bronchopneumonia or an acute exacerbation of chronic bronchitis were treated with amoxycillin 500 mg three times daily for 7 days. 26 were treated successfully. In 2 the response was rather slow. In a double-blind trial, a total of 116 patients were treated. They were divided into two groups of 58 patients, one group receiving amoxycillin 500 mg three times daily and the other, ampicillin 500 mg three times daily. The failure rate was 19 % in the amoxycillin group and 15.5 % in the ampicillin group. Analysis of the failures revealed that the amoxycillin group contained more seriously ill patients suffering from complications, especially congestive cardiac failure. Amoxycillin is free from toxicity and its side effects are no worse than those of ampicillin. It produces much higher serum and sputum levels than ampicillin. It is as effective as ampicillin in bronchopneumonia and probably more effective in acute bronchitis. In future studies the dose could be reduced.