We evaluated the usefulness of a rapid urinary antigen detection kit (Binax NOW) to detect Streptococcus pneumoniae in the early diagnosis of pneumococcal respiratory tract infections in 313 patients with presumptive respiratory tract infections. We compared results of this test with those of sputum Gram staining. Urinary antigen and sputum Gram staining were respectively positive in 37 and 36 of 57 patients with pneumococcal respiratory infections. The urinary antigen showed moderate positive rate of 64.9% and low false positive rate of 2.3%. The sputum Gram staining also showed moderate positive rate of 64.3% and low false positive rate of 3.5%. Pneumococcal antigen was more frequently detected in patients with severe pneumococcal infections (6/6) than those with mild (5/10) and moderate (26/41) infections. Of the 9 patients who had received antibiotics before testing, antigen was detected in 8 but positive results of sputum Gram stain were in 4. In conclusion, urinary antigen test is a useful test for early diagnosis of pneumococcal respiratory infections especially in adult patients with moderate or severe infections for whom demonstrative results of a sputum Gram stain is unavailable, even after commencement of antibiotic treatment.