The anaerobic threshold has generally been determined by simple visual inspection of ventilation or other gas-exchange data obtained during incremental exercise. To establish objective criteria for the determination of anaerobic threshold, a computer algorithm has been developed that models the ventilatory response to exercise using multisegment linear regression. The best-fit regression model is chosen by minimizing the pooled residual sum of squares . The anaerobic threshold is reported as the first break point in that model. The computer-determined anaerobic threshold values for 37 subjects were compared with subjectively determined values as chosen by four independent observers. The observers' estimates, when pooled to yield a single a single value for each subject, gave a mean value for the gas-exchange anaerobic threshold of 2.26 +/- 0.69 l/min. The estimates by the computer method averaged 2.21 +/- 0.65 l/min. The correlation coefficient for these two methods was 0.94.