The Bruce treadmill protocol is an often-used exercise test for children and adults. Few and mainly old normative data are available for young children. In this cross-sectional observational study we determined new reference values for the original Bruce protocol in children aged 4 and 5 years. Furthermore, we compared the original protocol with the so-called 'half Bruce' protocol. In the Netherlands this half-Bruce protocol is often used for young children because of the rather large increments in workload in the original protocol. Seventy-eight healthy Dutch children participated. The maximal endurance time was the criterion of exercise capacity. The new reference values for the original Bruce protocol are presented as reference centiles. The mean (SD) endurance time using the original protocol was 10.2 (SD 1.5) min.; this was 9.4 (1.3) min. for the half-Bruce protocol. The mean difference was 50 seconds (95% CI: 29-71 s, P < 0.001). So, for children aged 4 and 5 years the endurance times obtained with the original and half-Bruce protocol are different and should not be considered interchangeable. Our new reference values can be used as reference values for the original Bruce protocol.