Background: HIV/AIDS, a transmissible and deadly disease is now a chronic manageable disease. However, loss to follow-up (LTFU) is threatening its manageability by increasing the risk of drug resistance and transmission of resistant strains. Missing critical client information may not be random but predictive of LTFU. The study aims to determine the period prevalence, incidence rates, patterns, predictors and the predictive relationship of missing clinical information with LTFU in an HIV Treatment Program in a developing country. Methods: A 14-year retrospective analysis was conducted on routinely collected data validated using the Retention and Audit Determination Tool (RADET) from adult HIV clients who initiated treatment from 2008 to 2021 in the HIV treatment centre of Imo State University Teaching Hospital, Nigeria. LTFU was established when a client had not been seen at the clinic for at least 90 days after the last missed scheduled clinic appointment. Kaplan Meier (KM) curves, log-rank test and Cox proportional hazards regression analyses were used to determine the distributions of LTFU and predictors with the level of significance set at p≤ 0.05. Results: The 14-year period prevalence was 39.4% with an incidence rate of 13 new cases of loss to follow-up per 100 HIV clients per year. The majority of the HIV clients (59.3%) had at least one critical missing file information. LTFU were significantly more likely to occur in those aged less than 45 years old (aHR:1.38, p<0.001) who are males (aHR: 1.19, p=0.006), suspected or diagnosed with TB (aHR: 7.78, p<0.001) and weighing 70kg or less (aHR:1.79, p<0.001). Missing viral load information (aHR: 51.37 p<0.001) and missing HAART regimen line information (aHR: 8.44, p<0.001) were the most predictive of LTFU in the model. Conclusion: Though LTFU in HIV patients linked to treatment, appears to be reducing, it is still a significant threat in the management of HIV disease. Further reduction in LTFU could be achieved by institutionalizing biometric capture, improving HIV/TB services and training staff in HIV-enhanced adherence counselling services.