Background- ‘loss of follow-up’ is a significant public health concern globally. Retention in preventive care among at-risk key and priority population groups is critical for the successful prevention of HIV acquisition. Loss to follow-up of individuals has countless negative impacts on HIV prevention outcomes. There is, however, limited information about the incidence and predictors of loss to follow-up in the study area. Thus, this study aimed to determine the incidence rate and predictors of loss to follow-up among Prep users.Method- A retrospective cohort study was undertaken using 240 PrEP users between May 2020, and May 26, 2022, at Addis Ababa selected health centers. All eligible clients who fulfilled the inclusion criteria were included in study.Results and discussionA total of 240 participants with a median age of 32.0 years (interquartile range [IQR]: 27.0 to 40.0) had a median time since initiation of PrEP 21.2 months (IQR: 6.5 to 22.1). Almost half 122 (50.8%) were married, single 58 (24.2%), divorced 10 (4.2%), and widowed 50 (20.8%). One third were female sex workers (33.3%), and the rest (66.7%) were sero-discordant couples. Most of the users were from ART clinics (60.8%) and (39.2%) were from PMTCT clinics Being male is associated with 1.77 times in risk of getting lost than female (ARR=1.77, 95% CI =1.12-2.79). those who had no adherence counseling was associated with 1.86 times in the chance of LTFU as compared to those who had have adherence counseling (ARR=1.863, 95% CI =1.184, 2.930, P-value=0.003). 86 (35.8%) experienced loss to follow-up and the overall incidence rate of loss to follow-up was 7.3 (95% CI: 4.3-12.6) per 100 person-years of observation (PYs). PrEP adherence strategies should be developed and designed as a holistic approach, acknowledging the contextual factors of key population groups.Conclusion- About 35.8% clients became LTFU of PrEP with overall incidence of 7.3(4.3-12.6) per 100 pys observation. Research preparedness involving key and priority population groups should be strengthened for HIV prevention intervention evaluations in Ethiopia.