A break point in the double product versus work rate relationship (DPBP) during incremental exercise has previously been reported. The aim of the current study was to investigate the frequency and degree of inter-observer agreement with which a DPBP could be detected. We also wished to determine its relationship, if any, to the lactic acidosis threshold (LATla). Ten normal volunteers performed continuous incremental bicycle ergometer exercise under three different concentrations of inspired oxygen (FiO2 = 0.21, 0.15 and 0.12). In addition, a group of patients with diseases expected to result in impaired exercise tolerance performed exercise under room air conditions. Gas exchange was measured continuously and double product was measured at 15-s intervals throughout exercise using an automated sphygmomanometer. Four observers, unaware of subject identity and test condition, analysed a total of 39 tests. All four observers detected a DPBP in 29 cases (74%) and an LAT in 35 cases (90%). The intra-class correlation coefficient was 0.76 for the DPBP and 0.93 for the LATla, indicating a reasonable level of agreement among observers for both break points. The DPBP was closely related to the LATla (r = 0.865, P < 0.0001), although it occurred at a slightly higher oxygen consumption (VO2, bias 0.137 1.min-1, 95% confidence intervals 0.041-0.233). We conclude that the DPBP is a common occurrence during incremental exercise. The close relationship between the DPBP and the LATla suggests that both may reflect similar events at the level of the skeletal muscle cell.