Reduced intestinal CD4 T cell numbers and gastrointestinal disease are common features of acquired immunodeficiency syndrome (AIDS). Duodenal lymphocyte densities and mucosal addressin cell adhesion molecule (MAdCAM)-1 expression were analyzed in patients with AIDS after highly active antiretroviral therapy (HAART). Compared with human immunodeficiency virus (HIV)-seronegative individuals, HAART-naive patients with AIDS displayed reduced duodenal CD4 T cell densities. After HAART, AIDS patients with opportunistic intestinal pathogens displayed greater increases in duodenal lamina propria (LP) CD4 T cell densities than patients without such infections. Duodenal MAdCAM-1 expression was elevated in all HAART-naive patients with AIDS but remained elevated only in the intestinal pathogen group after HAART. The data suggest that, in HIV-1 infection, lymphocyte migration to the intestine may be promoted by increased MAdCAM-1 expression. After HAART, opportunistic intestinal pathogens maintain elevated MAdCAM-1 expression, which results in prominent increases in LP CD4 T cell densities in the absence of HIV-mediated CD4 T cell destruction.