Capillary blood lactate assessment is increasingly used by well-trained runners to monitor the intensity of endurance exercise. In order to examine the known association of exercise intensity with blood lactate accumulation also in less trained subjects, we analysed data from a standardized incremental maximal test on the treadmill of 319 men (age 22.9 +/- 5.5 years, [means +/- S.D.]) and 145 women (22.7 +/- 4.5 years) characterized by a wide variation in endurance capacity. Results showed that the running velocity eliciting a blood lactate concentration of 4 mmol/l did not correspond to the same exercise intensity in well endurance-trained vs poorly endurance-trained subjects. At 4 mmol/l blood lactate, the slowest decile of men (i.e. 32 out of 319) ran at 71 +/- 4.7% (corresponding to 2.9 +/- 0.3 m/s) of their maximal treadmill velocity attained during the test (4.1 +/- 0.4 m/s), indicating a rating of perceived exertion (RPE) of 12.3 +/- 1.8 points (Borg scale, range 6-20 points), while the fastest decile of men (n = 32) ran at 91 +/- 3.1% (corresponding to 5.4 +/- 0.2 m/s) of their maximal treadmill speed (5.9 +/- 0.2 m/s), indicating a RPE of 16.6 +/- 1.1 points. Very similar results were observed in women. There was a highly significant, positive correlation between running speed eliciting a blood lactate concentration of 4 mmol/l and RPE when running at this speed, with r = 0.64 in men and r = 0.55 in women. At the same proportional level of maximal running velocity, poorly endurance-trained athletes showed a 2-3 mmol/l higher capillary lactate concentration than well endurance-trained athletes, with both groups indicating the same RPE. These results suggest that fixed blood lactate concentrations not at all mean the same exercise intensity for well vs poorly endurance-trained subjects; this systemic trend should be considered when using blood lactate assessment for individual exercise counselling.