By the end of 2012, Zimbabwe had enrolled more than half a million patients on free antiretroviral treatment (ART). Reviews of ART programmes in sub-Saharan Africa revealed that one third of the patients drop out of the programme during the first three years of treatment. I explored reasons for programme attrition in an NGO-supported ART programme in rural Zimbabwe. In a mixed-methods design, I combined a retrospective cohort study among 8425 adult patients who initiated ART from 2005 to 2012, with a qualitative inquiry including 44 interviews, 8 focus group discussions with 78 participants, and participant observation during two months of field work. Programme retention at three years was 75%. Tracing of patients lost to follow-up revealed that mortality at 12 months was 1.8 times higher than reported before tracing: corrected mortality was 11.5%, 18.8% and 21.3% after 12, 36 and 60 months.574/1055 (54. 4%) deaths occurred before the next ART appointment, 481/1055 (45.6%) after missing the appointment. Programme retention did not imply consistent adherence to ART: 1780/6291 (28.3%) retained patients temporarily interrupted ART at least once for more than seven days. Qualitative data showed that patients temporarily interrupted ART for different reasons rooted in social, cultural and gender conflicts and related to mobility. ART restart was facilitated by social capital in family and kinship networks, decentralised access to ART and trusting patient-health worker relationships. Social isolation and discrimination of HIV-positive women by partner and in-laws, health workers blaming patients for missing an ART appointment and distance to the initiation site were the main reasons for prolonged and potentially fatal ART discontinuation. Our findings suggest that ART re-engagement after interruption is key for long-term retention. Programmes can prevent attrition from ART by decentralising their services, supportive management by health workers, and early tracing of medically- and socially vulnerable treatment interrupters.