Observational studies have found that women tend to have lower adherence to highly active antiretroviral therapy (HAART) than men do, though no meta-analysis has yet investigated this trend. The aims of the current meta-analysis are to determine if and to what degree the percentage of men versus women maintaining ≥90% adherence to prescribed HAART differs, and if the external variables moderating adherence differs by gender. Eight electronic databases were searched to locate all relevant studies available by May 2011. Fifty-six observational studies were eligible for inclusion in the meta-analysis. A random effect model was assumed for the global percentage estimation and to explain the heterogeneity. Across these studies, the difference between men and women in the proportion of individuals with ≥90% adherence to HAART was marginally significant (p<0.1; 67% and 62%, respectively). A greater proportion of men maintaining ≥90% adherence to HAART was more likely in studies with higher proportions of men who have sex with men (MSM), lower proportions of male alcohol users or lower proportions of men in a methadone program. In women, higher rates of adherence were found in studies conducted in Africa, Asia, and South America, when the sample included more widows or when the sample had a lower basal CD4 count. That both the percentage of adherent individuals and the variables associated with such adherence differ between men and women are suggestive of the need for improving gender-tailored interventions for adherence to HAART.