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BACKGROUND Diagnostic and treatment delays increase the severity and transmission of pulmonary tuberculosis (PTB). This study aimed to evaluate TB diagnostic and treatment delays in acid-fast bacilli (AFB) smear-negative patients. METHODS This was a retrospective observational study. Patients with positive AFB culture of Mycobacterium tuberculosis complex (MTC) were selected from among hospitalised patients with a diagnosis of pneumonia. Admission ward, anti-tuberculosis treatment and the duration of AFB culture were compared between smear-positive and smear-negative patients. RESULTS Of the 70 patients with positive isolation of MTC in AFB culture, 27 (38.5%) were smear-negative; of these, 18 (66.7%) were not isolated while in hospital, and 17 (63%) were neither diagnosed nor treated for TB. In contrast, 41 of the 43 smear-positive patients (95.3%) were directly admitted or quickly transferred to the isolation room and started on anti-tuberculosis treatment (P < 0.001). Samples from smear-negative patients required more time to grow MTC in AFB culture than those of smear-positive patients (23 days vs. 14 days, P < 0.001). Diabetes was significantly associated with AFB smear positivity, with an odds ratio of 12.2. CONCLUSIONS Negative AFB smears caused significant diagnostic and treatment delay. Patients staying in the general ward were exposed to TB patients who were not diagnosed in time.