IntroductionUnsuccessful TB treatment outcome is a serious public health concern. It is compelling to identify, and deal with factors determining unsuccessful treatment outcome. Therefore, study was aimed to determine pattern of unsuccessful TB treatment outcome and associated factors in eastern Ethiopia.MethodsA case control study was used. Cases were records of TB patients registered as defaulter, dead and/or treatment failure where as controls were those cured or treatment complete. Multivariate logistic regression models were used to derive adjusted odds ratios (OR) at 95% CI to examine the relationship between the unsuccessful TB treatment outcome and patients’ characteristics.ResultsA total of 990 sample size (330 cases and 660 controls) were included. Among cases (n = 330), majority 212(64.2%) were because of death, 100(30.3%) defaulters and 18(5.5%) were treatment failure. Lack of contact person(OR = 1.37; 95% CI 1.14-2.9, P, .024), sputum smear negative treatment category at initiation of treatment (OR = 1.8; 95% CI 1.3-5.5,P, .028), smear positive sputum test result at 2nd month after initiation treatment (OR = 14; 95% CI 5.5-36, P,0.001) and HIV positive status (OR = 2.5; 95% CI 1.34-5.7, P, 0.01) were independently associated with increased risk of unsuccessful TB treatment outcome.ConclusionDeath was the major cause of unsuccessful TB treatment outcome. TB patients do not have contact person, sputum smear negative treatment category at initiation of treatment, smear positive on 2nd month after treatment initiation and HIV positive were factors significantly associated unsuccessful treatment outcome. TB patients with sputum smear negative treatment category, HIV positive and smear positive on 2nd nd month of treatment initiation need strict follow up throughout DOTs period.