This meta-analysis synthesizes eighty-four observational studies, conducted across twenty countries, to determine the mean proportion of people who reported ≥90% adherence to prescribed highly active antiretroviral therapy (HAART) and to identify the factors associated with high levels of adherence. Eight electronic databases were searched to locate all relevant studies available by January 2010, which were then coded for sample characteristics and adherence levels. The average rate of reporting ≥90% adherent HAART adherence is 62%. However, this proportion varies greatly across studies. In particular, a greater proportion of individuals maintaining ≥90% adherence to HAART is more likely in studies with higher proportions of men who have sex with men (MSM) and lower proportions of injection drug users (IDU), with participants in an earlier stage of infection, and in studies conducted in countries characterized by lower Human Development Index (HDI) scores.