For maximum suppression of the multiplication of the human immunodeficiency virus (HIV) virus and subsequent increase in the CD4 cell count, a level of adherence of 95% and above is required among the patients on Antiretroviral drugs (ARVs). Most patients on antiretroviral therapy (ART) in Nairobi are not achieving the optimum adherence level required to maintain treatment efficacy, hence the risk of drug resistance and increased burden in the public health care system. The aim of this study was to identify the factors that influence adherence to ART among HIV patients attending public and private health facilities in Nairobi, Kenya. A non-interventional cross sectional study using both qualitative and quantitative data collection methods was used. The study was carried out in the Public and Private Health facilities offering ART in Nairobi, Kenya. Four hundred and fifty People living with HIV and acquired immune deficiency syndrome (AIDS) (PLWHA) receiving ARVs in selected public and private health facilities in Nairobi, between June, 2007 to June, 2008 were selected for the study. The composite adherence ART level among patients in Nairobi was found to be 85%. The major factors that were found to constrain adherence were costs, lack of social support, side effects, time to reach the health facility, and adequate knowledge of ARVs. This study found out that majority of the patients on ART in Nairobi are not achieving optimum adherence. The major factors that lead to the sub-optimal adherence are lack of social support, lack of disclosure that one is taking ARVs, poor knowledge of ARVs, associated costs such as transport and extra food requirements and the existence of side effects.