The present study tested the hypothesis that performance would improve when the work rhythm of a highly repetitive task was synchronous with a worker's internal physiological rhythms. Experienced office workers (n = 20) used video display terminals (VDTs) to perform a repetitive, self-paced data-entry task in a simulated office environment over a 2-day period. Each work day consisted of six 40-min work periods. Work rhythm changes were induced by varying input data field lengths (3-13 characters) across eleven of the twelve work periods. The degree of synchronization between the work and breathing rhythms, and also between the work rhythm and variations in the interbeat interval, was scored using cross-spectral analysis. Synchronization scores were then used to predict keying performance using multiple regression analysis. The degree of synchronization between the work and breathing rhythms was not predictive of performance. However, increased synchronization between the work and cardiac rhythms was predictive of (a) increased keystroke output, (b) lower error rate and (c) lower correction rate. The results suggest that performance in repetitive VDT work might improve if the task is designed to promote work-physiological synchronization.