The combination of sulphadiazine 225 mg (SD) and trimethoprim 75 mg (TMP) was compared with the combination of sulphamethoxazole 400 mg (SM) and trimethoprim 80 mg (TMP) in the treatment of acute respiratory tract infection using a randomized double-blind clinical trial. The dose of both preparations was two tablets twice daily for 7–14 days. The patients were 200 conscripts hospitalized for an acute respiratory infection warranting antibacterial treatment. The most frequent diagnoses were pneumonia, bronchitis and tonsillitis. The combination SD/TMP was statistically significantly (p < 0.001) superior to the combination SM/TMP regarding overall efficacy of all indications. Especially pneumonia and bronchitis responded more favourably to SD/TMP than to SM/TMP. The duration of fever was also shorter (p < 0.001) with SD/TMP than with SM/TMP. The occurrence of side effects was 2% in both treatment group