Of 1,710 Chinese patients with radiologically active pulmonary tuberculosis but with sputum negative for acid-fast bacilli on four or more initial microscopic examinations who were studied for 5 yr, 592 (35%) had one or more initial sputum cultures positive for Mycobacterium tuberculosis. These 592 patients were randomly allocated to receive streptomycin, isoniazid, rifampin, and pyrazinamide daily for 4 months or 3 times a week for either 4 or 6 months. The remaining 1,118 patients with all their initial cultures negative were randomly allocated to receive the same four drugs daily for 3 months or 3 times a week for either 3 or 4 months. There were no bacteriologic failures during chemotherapy, and the relapse rates for the 4-month regimens during the 5 yr were 2% in 293 patients with drug-susceptible cultures initially (95% confidence limits, 1 to 5%); 8% in 59 patients with cultures resistant to isoniazid, streptomycin, or both drugs, but susceptible to rifampin initially; and 4% in 325 patients with all their cultures negative initially (95% confidence limits, 1 to 7%). The combined relapse rate for the 3-month regimens was 7% in 709 patients with all their cultures negative initially (95% confidence limits, 5 to 9%). In Hong Kong, 4 months of chemotherapy is now used routinely in the treatment of patients with smear-negative pulmonary tuberculosis, whether their initial sputum cultures are positive or negative.