HIV infection leads to severe depletion of CD4+ counts with subsequent reduced levels ofrncirculating CD4+ lymphocytes in the peripheral blood. CD4+ counts are the primary laboratoryrnmarkers used to track the progression of HIV to AIDS. It is the strongest predictor for risk of deathrnand AIDS (DHHS, 2011) at the time of initiating therapy and used to decide when to start ARTrntreatment. The main objective of this study was to model the progression of CD4+ cell counts ofrnHIV- positive patients following ART. A longitudinal data was obtained from ART case unit ofrnHIV positive patients in Yekatit 12 Hospital who were following ART from September 10, 2009rnto September 9, 2012 G.C and followed until January 9, 2016 G.C, Addis Ababa. The data wasrnextracted from SMART CARE of patients chart and patients with at least three times ofrnmeasurements and those in first line ART regimens were included. The linear mixed modelrnanalysis was applied taking the correlation between CD4+ counts of patients into account. Wernincluded 217 patients of whom 141 (65%) were females and only 51.61% of patients were followedrnuntil the last follow up time inducing 48.39% missingness. The results from the linear mixed modelrnwith AR(1) covariance structure showed that baseline CD4+ count, WHO clinical stage, time andrnthe interaction effects of sex, Age, baseline CD4+ count and regimen class with time werernstatistically significant factors for the progression of CD4+ count at a square root level over time.rnResults from the full likelihood and multiple imputation approaches were found to be almostrnidentical for covariates that were significant. The mean CD4+ count showed an increasing progressrnover time after patients were initiated on ART. We recommend that patients should be initiated tornART regimens at an early WHO clinical stages, at a higher CD4+ count and ART regimens shouldrnbe taken continuously to increase the progress of CD4+ count.rnKEY WORDS: ART, CD4+, Linear Mixed Model, Longitudinal Data, Missing Data, AR(1), ML,rnMI, MAR