The ventilatory response to graded treadmill exercise was studied in 50 children with congenital heart disease (CHD), a left-to-right intracardiac shunt, and compared to that of age and sex-matched healthy children. In all these children, a breakpoint in the minute ventilation (VE) in relation to increasing oxygen uptake (VO2) was detected. Because of the reported coincidence of the nonlinear increase in VE and lactate with increasing VO2, this threshold has been named "ventilatory anaerobic threshold" (VAT), expressed in ml O2 X min-1 X kg-1. VAT correlated well with VO2 (ml X min-1 X kg-1), reached at a heart rate of 170/min (VO2,170). Compared to normal children, however, a significantly (p less than 0.05) larger percentage (56%) of the patients showed a subnormal value (below the 95% confidence limit) for VAT than for VO2,170 (28%). Furthermore, when expressed as a percentage of the mean normal value, the mean VAT (89 +/- 14.4% SD) was significantly (p less than 0.05) lower than the mean VO2,170 (103 +/- 17.2%). VAT (ml O2 X min-1) correlated significantly with body weight and height, even after adjusting for age, and with the percentiles of body weight and height. VAT constitutes a useful noninvasive criterion for evaluating exercise performance in children with CHD, at least those forms with a left-to-right intracardiac shunt, and it is significantly more sensitive than the VO2,170 in detecting decreased cardiorespiratory endurance capacity.