We identify factors associated with the normalization of the CD4+:CD8+ T cell ratio among UK Collaborative HIV Cohort study participants, and describe the association of the CD4+ and CD8+ T cell counts and the CD4+:CD8+ T cell ratio, with the risk of new AIDS events among individuals who achieve a suppressed viral load. Participants initiating combination antiretroviral therapy (cART) after 2006 with a CD4+:CD8+ T cell ratio <1, and viral suppression within 6 months were included. Cox proportional hazard models were used to examine associations with ratio normalization (ratio ‡1). Poisson regression models were used to investigate factors associated with the development of AIDS after viral load suppression. A total of 13,178 participants [median age: 37 (interquartile range: 31-44)] were followed for 75,336 person-years. Of the 4,042 (32.9%) who experienced ratio normalization, individuals with a high CD4+ T cell count [>500 vs. £200 cells/mm 3 , adjusted hazard ratio (95% confidence interval): 7.93 (6.97-9.01)], low CD8+ T cell count [>1,150 vs. £500 cells/mm 3 : 0.18 (0.16-0.21)], and low CD4+:CD8+ T cell ratio [>0.8 vs. <0.2: 12.36 (10.41-14.68)] at cART initiation were more likely to experience ratio normalization. Four hundred and nineteen people developed a new AIDS event. Most recent CD4+ T cell count [>500 vs. £200 cells/mm 3 : adjusted rate ratio 0.24 (0.16-0.34)] and CD4+:CD8+ T cell ratio [>0.8 vs. <0.2: 0.33 (0.21-0.52)] were independently associated with a new AIDS event. One third of study participants experienced ratio normalization after starting cART. CD4+ T cell count and CD4+:CD8+ T cell ratio are both individually associated with ratio normalization and the development of new AIDS events after cART.