Mortality assessment in cohorts with high lost to follow-up (LTFU) is challenging in settings with limited civil registration systems. We aimed to assess mortality in a clinical cohort (KIULARCO) of HIV-infected persons in rural Tanzania, accounting for unseen deaths among participants LTFU. We included adults enrolled in 2005-2015 and traced a non-random sample of those LTFU. We estimated mortality using Kaplan-Meier methods with: A) routinely-captured data; B) crudely incorporating tracing data; C) weighting using tracing data to crudely correct for unobserved deaths among participants LTFU; and D) weighting using tracing data accounting for participant characteristics. We investigated associated factors using proportional hazards models. Among 7460 adults, 646 (9%) died, 883 (12%) transferred clinics, and 2911 (39%) were LTFU. Of 2010 (69%) traced participants, 325 (16%) were found: 131 (40%) died and 130 (40%) transferred. Five-year mortality estimates were A) 13.1%; B) 16.2%; C) 36.8%; D) 35.1%. Higher mortality was associated with male sex, referral as hospital in-patient, living close to the clinic, lower body mass index, more advanced WHO stage, lower CD4 count, and less time since antiretroviral therapy initiation. Adjusting for unseen deaths among participants LTFU approximately doubled the five-year mortality estimates. Our approach is applicable to other cohorts adopting targeted tracing
