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Human immunodeficiency virus type 1 (HIV-1) RNA and p24 antigen concentrations were determined in plasma samples from 169 chronically infected patients (median CD4 cell count, 140 cells/μL; range, 0-1500 cells/μL). p24 quantification involved heat-mediated immune complex dissociation and tyramide signal amplification—boosted ELISA, which has a diagnostic sensitivity similar to that of RNA quantification by a commercial polymerase chain reaction kit. In Cox's proportional hazard models adjusted for CD4 cell count, both RNA (P < .005) and p24 (P = .043) levels were significant predictors of progression to AIDS. Measurement of p24 was superior to measurement of RNA in the model for survival (P = .032 vs. P = .19). p24 level was a significant predictor of CD4 cell decline in models adjusted for CD4 cell counts and was superior or equivalent to RNA level, depending on the group analyzed. Stratification by CD4 cell counts at baseline showed that the superiority of p24 measurement was more pronounced at lower levels of CD4 cells (<200/μL). p24 level may be of interest as a simple and inexpensive predictive marker of disease progression.