Background: Post-infectious bronchiolitis obliterans (PIBO) is a rare form of chronic obstructive lung disease in children, with insufficient data on the factors that predict the prognosis of PIBO. Objectives: The aims of this study were to assess the long-term outcome of PIBO patients, evaluate prognostic factors, and investigate the relation between serum Krebs von den Lungen 6 (KL-6) level and disease severity. Methods: Thirty-eight children with a clinical and radiological diagnosis of PIBO were enrolled on this study. Information was obtained from medical records regarding the clinical characteristics related to the first severe lung infection, the patient's clinical presentation at PIBO diagnosis, treatment, and overall outcome. The clinical conditions of the patients at the time of their PIBO diagnosis and at their last follow-up admission were evaluated using the clinical severity score (CSS). Samples for serum KL-6 level were obtained in 36 PIBO and 42 control group patients. The association of KL-6 level and PIBO severity was analyzed. KL-6 was also compared with the control group. Results: The median age of the 38 PIBO patients was 8.3 years (range 1.4-17.1 years), and the patients were monitored for a median of 52.5 months (range 1-180 months). The last CSSs of patients who were mechanically ventilated at first severe lung infection were greater than others (p = 0.033). First and last CSSs were significantly correlated (r = 0.355, p = 0.029). Serum KL-6 levels of PIBO patients were between 2 and 321.73 IU/mL (median 4.87 IU/mL). Serum KL-6 levels of PIBO patients were significantly lower than in the control group (p <= 0.01). Patients hospitalized for a longer time at the first severe pulmonary infection had significantly lower KL-6 levels at their last control (r = -0.498, p = 0.004). Conclusions: PIBO patients who were mechanically ventilated during the first severe lung infection had worse clinical outcomes after long-term follow-up. Serum KL-6 concentrations in PIBO patients were lower than in the control group. PIBO patients who were hospitalized and mechanically ventilated at first severe pulmonary infection were prone to having lower KL-6 concentrations.