Background: Non-adherence to treatment poses a challenge to tuberculosis (TB)treatment since it increases the risk of drug resistance, death, relapse and prolongedinfectiousness. TB patients co-infected with human immunodeficiency virus (HIV)constitute a large proportion of TB patients in Ethiopia. How being co-infected with HIVas well as concomitant treatment with anti-retroviral therapy (ART) among other thingsaffect adherence to TB treatment has not been studied in Ethiopia, and we did a study toexplore this.Objectives: To assess magnitude of default and factors associated with defaulting fromTB treatment in patients with TB/HIV co-infection who received concomitant treatment,and to assess co-infected patients’ and health professionals’ perspectives of barriers andfacilitators of TB treatmentMethods: Through a review of records, we retrospectively assessed the TB treatmentoutcomes in a cohort of TB patients co-infected with HIV who were initiated on TBtreatment in the period July 2006-September 2007 and who concomitantly received ARTin three Health Centers in Addis-Abeba. We conducted in-depth interviews with 15purposefully selected patients and 9 health professionals involved in their management.We also conducted two focus group discussions in patients with TB/HIV co-infectionreceiving concomitant treatment.Results: We found that, from the 248 patients included, 148(59.7%) completed treatment,32(12.9%) were cured, 31(12.5%) died, twenty six patients (10.5%) defaulted treatment,3(1.2%) failed and 8 patients (3.2%) were transferred out. Multiple regression analysisshowed that male sex (AOR= 2.66(95%CI: 1.05- 6.71), P=0.038), lower education(AOR= 3.81(95% CI: 1.38- 10.52), P= 0.009), and timing of ART initiation wereassociated with defaulting from TB treatment, with those started on ART in the intensivephase having the highest risk of defaulting (AOR= 9.68(95%CI: 2.71- 34.57), P= <0.001,as compared to those initiated on ART in the continuation phase), followed by those6initiated on ART before anti-TB initiation (AOR= 4.83(95% CI:1.19- 19.83), P= 0.025,as compared to those initiated on ART in the continuation phase) . The qualitative studyfurther showed that many misperceptions exist among patients about TB/HIV, and thatadverse perception of prognosis of TB in the presence of HIV with an eagerness tosurvive, support from families and health professionals were positively influencingtreatment, whereas side effects, pill burden, economic constraints, lack of food, stigma,lack of adequate communication with health professionals and poorly organized TB/HIVcollaborative activities were acting as barriers to treatment.Conclusion: The default rate from TB treatment among patients who are TB/HIV coinfectedand who received concomitant treatment is still higher than the World HealthOrganization recommended rate, and many of those who finish treatment seem to facemany difficulties. Health professionals and policy makers should be aware of factorsinfluencing TB treatment in these patients and act accordingly, since many of thesefactors are amenable to change.