During a phase 2 trial of parainfluenza virus type 3 (PIV3) vaccine, sequential serum samples were obtained from infants at 2, 6, 7, 12-15, and 13-16 months of age. Paired serum samples obtained at 2 and 6 months of age were used to estimate the biologic half-life of human PIV3 (hPIV3) maternal antibody in young infants. On the basis of the assumption that hPIV3 maternal antibody decays exponentially and constantly, the biologic half-life was estimated without adjusting for body weight increases. Cumulative proportions of hPIV3 infection in young infants were further estimated after adjusting for maternal antibody decline. A hemagglutination inhibition assay was used to quantify hPIV3 antibody. The mean (95% confidence interval) biologic half-life was estimated to be 51 (42-60) days, on the basis of which cumulative proportions of hPIV3 infection were estimated to be 11% at 6 months of age, 47% at 12-15 months of age, and 50% at 13-16 months of age.