High levels of adherence to antiretroviral therapy (ART) are necessary for achieving and maintaining optimal virological suppression, as suboptimal adherence leads to therapy failure and disease progression. It is well known that adherence to ART predicts therapy response, but it is unclear whether clinical outcomes of ART predict adherence. To examine the predictive power of current CD4+ T cell count for adherence of HIV-infected individuals to ART, we performed a cross-sectional analysis of 133 Dutch HIV patients with electronically measured adherence. In a multivariate analysis adjusting for a number of sociodemographic and clinical variables, high current CD4+ T cell count (>660 cells/mm3) was most strongly associated with lower adherence to ART (assessed as a continuous variable) during a two-month period immediately following the measurements of variables (P = 0.008). The twice-per-day (versus once-per-day) dosing regimen was also significantly associated with lower adherence (P = 0.014). In a second multivariate analysis aimed at determining the predictors of suboptimal (<100% of the doses taken) adherence, high current CD4+ T cell count was again the strongest independent predictor of suboptimal adherence to ART (P = 0.015), and the twice-per-day dosing regimen remained associated with suboptimal adherence (P = 0.025). The association between suboptimal adherence and virological suppression was significant in patients with high CD4+ T cell counts, but not in patients with low or intermediate CD4+ T cell counts (P = 0.036 and P = 0.52, respectively; P = 0.047 for comparison of the effects of adherence on virological suppression between patients with high vs. low or intermediate CD4+ T cell counts), suggesting that apart from promoting suboptimal adherence, high CD4+ T cell count also strengthens the effect of adherence on virological suppression. Therefore, sustained efforts to emphasize continued adherence are necessary, especially for patients with high CD4+ T cell counts.