Even though the world is ghting HIV disease in unity and patients are getting antiretroviralrntherapy treatment, HIV disease continues to be a serious health issue for parts of the world andrnlarge number of AIDS related deaths are being registered every year. A number of studies havernbeen conducted to assess factors related with the progression of the disease using surrogate end-rnpoints like CD4 cell count. The main objective of this study was to make use of appropriaternstatistical models to analyze CD4 cell counts data and identify associated risk factors a ectingrnthe CD4 cell progression of patients under ART tra etment in Debre Markos Re eral Hospital. Inrnthis longitudinal retrospective cohort based study, data was collected from 445 HIV patients reg-rnistered for ART treatment between September, 2005 and August, 2014 in the Hospital. Poisson,rnPoisson-Gamma, Poisson-Normal, and Poisson-Gamma-Normal models were applied to accountrnfor overdispersion and correlation in the data. Poisson-Gamma-Normal model with random in-rntercept was selected as a best model to t the data based on di erent model selection criteria.rnThe ndings of the study revealed that time in months, sex of patients, baseline WHO stagernand baseline CD4 cell count were found to be signi cant factors for progression of HIV patients'rnCD4 cell count. Patients who started ART at higher baseline CD4 counts evolved higher thanrnthose who started at lower CD4 counts. Therefore, patients should start ART treatment early tornincrease their CD4 cell count progression.rnKeywords: CD4 count, Longitudinal data analysis, Poisson-Normal Model, Poisson-Gamma-rnNormal model, Antiretroviral therapy (ART), HIV/AIDS