Early studies of highly active antiretroviral therapy (HAART) use in clinical practice suggested suboptimal rates of viral suppression. HAART regimens and expertise in the use of HAART have since evolved, and we sought to determine how virologic response to HAART has changed in clinical practice. We compared all patients who started a first HAART regimen from 1996 through 2002 in a longitudinal cohort of HIV-infected patients in care in Baltimore. There were significant improvements in suppressing HIV RNA to < 400 copies/mL, ranging from 43.8% (1996) to 72.4% (2001-2002) by 6 months and from 60.1% (1996) to 79.9% (2001-2002) by 12 months (both P < 0.01 for trend). There were also significant improvements in CD4 cell response. Over time, there was a significant increase in the use of a nonnucleoside reverse transcriptase inhibitor (NNRTI) or boosted protease inhibitor (PI) regimen compared with a single PI as well as an increase in the number of patients who were antiretroviral (ARV) naive. There was also a significant temporal trend from 1996 through 2002 in achieving a suppressed HIV RNA level, adjusting for being ARV naive, specific HAART regimen, CD4 cell count, HIV-1 RNA level, and demographic factors. This suggests that improved virologic response may also be attributable to other factors such as a greater focus on medication adherence, improved ARV tolerability, and ease of dosing.