The relationship between 12-month CD4(+) cell count response and clinical outcome (AIDS-defining event or death) in a subset of 228 patients with a human immunodeficiency virus load >400 copies/mL despite receiving combination antiretroviral therapy as part of a larger randomized trial was defined by use of Cox models. The 12-month CD4(+) cell count responses were divided into 5 categories, ranging from decrease or no change (29% of patients) to a > or =100-cell/mm(3) increase (27% of patients). There was a lower risk of clinical progression for each incremental increase in CD4(+) cell count response. A 25-cell/mm(3) increase in CD4(+) cell count was associated with a 21% reduction in the risk of an AIDS-defining event or death (P<.0001).