The objective of this study was to determine if a lower rate of adherence (<95%) is sufficient to maintain HIV viral suppression in patients on an efavirenz-based regimen. This study was a retrospective review of pharmacy refill records at an HIV specialty pharmacy at Montefiore Medical Center's outpatient clinic. Data from 151 HIV-positive patients on an efavirenz-based regimen with at least one undetectable viral load (HIV RNA < 400 copies/mL) from December 2003 through March 2005 were reviewed. Adherence was calculated based on the formula: [(pills dispensed/pills prescribed per day/days between refills)x100%]. Calculated adherence for each time-period was correlated to the respective HIV-RNA value for that period. Of 151 patients, viral suppression was maintained in greater than 80% of time periods for adherence rates as low as 85-90%. The periods with 75-80% adherence also had higher than 85% suppression. Rates of suppression began to fall when adherence decreased to < 75%. In conclusion, lower adherence rates (<95%) on an efavirenz-based regimen were more successful in maintaining viral suppression than previously found with unboosted protease inhibitor-based regimens.