In an overall of the value of an antibiotic it is important to take into account the onset of bacterial resistance in addition to its immediate clinical effects since the former can render an otherwise efficacious antibiotic obsolete within a short space of time. We studied two groups of intubated patients with pneumonia infections during two different periods in 1990: January-June and October-December; each patient was treated with pefloxacin 400 mg twice a day i.v. for 6 days on the basis of an antibiogram carried out on tracheobronchial secretion; at the end of that period patients were clinically evaluated and a second bacteriological test was performed. It was found that the percentage of pefloxacin-resistant bacterial strains did not increase significantly after this antibiotic had been used for a year for pneumonia infections (from 27.5% to 33%). The appearance of bacterial resistance at the six-day treatment cycle was similar in both groups (19% and 22%). Of all the bacterial strains, Pseudomonas aeruginosa showed the greatest tendency to become resistant.