Introduction: Treatment outcome is an important indicator of tuberculosis control programs, as suggested by the World Health Organization. However, this has not been well documented in the study area. This work contributes to a better understanding this issue. Methodology: A five-year (2009-2013) retrospective cohort study was conducted between April and May 2014, in six randomly selected health institutions providing tuberculosis treatment in western Ethiopia. Bivariate and multivariate logistic regression analyses were used to assess the association between treatment outcomes and predictor variables. Results: A total of 1,175 tuberculosis patients with a mean (standard deviation) age of 29.91 (13.99) were involved in the study. The majority of the study participants had smear-negative pulmonary tuberculosis (39.7%) and extrapulmonary tuberculosis (39.7%). Of all the study participants, 14.5% were cured, 56.3% completed treatment, 0.2% had treatment failure, 8.1% died during follow-up, 7.1% were reported as defaulters, and 13.8% were transferred out to another health institution. The overall treatment success rate was 70.8% and show progressive increases over the course of the study. The associated predictors were enrollment years, HIV co-infection, and sputum smear follow-up in the second, fifth, and seven months. Conclusions: The treatment success rate was unsatisfactory in spite of improvement seen over the study period. Thus, continued follow-up of patients, with frequent supportive supervision during the course of treatment, and provision of early detection and follow-up for HIV infection need to be strengthened to achieve an effective treatment outcome.