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We investigated the impact of neutralizing antibodies (NAbs) on CD4 T-cell count and viral load in a cohort of HAART recipients who underwent extended structured treatment interruption.Substudy of NAb in the AIDS Clinical Trials Group 5170 trial.Early plasma samples from 50 volunteers who discontinued HAART were evaluated in a peripheral blood mononuclear cell-based neutralization assay against a panel of four subtype B primary isolates.We found that high-titer (90% inhibitory dose > 500) NAb against two or more isolates was associated with reduced viral load (P = 0.003 at 12-week posttreatment interruption). This effect faded with time, losing significance (P = 0.161) by study conclusion. Participants possessing the highest NAb levels against individual isolates appeared more likely to have lower viral loads with the association gaining significance against the R5-tropic primary isolate US1 (P = 0.005). There was no association between broader neutralization and CD4 T-cell slope over time.The data suggest that high-titer NAb responses at the time of treatment interruption are associated with reduced viral load over time, but not CD4+ T-cell decline.