PYNE, D. B., H. LEE, and K. M. SWANWICK. Monitoring the lactate threshold in world-ranked swimmers. Med. Sci. Sports Exerc., Vol. 33, No. 2, pp. 291–297, 2001. To determine whether lactate profiling could detect changes in discrete aspects of endurance fitness in world-ranked swimmers during a season. Eight male and four female Australian National Team swimmers aged 20–27 yr undertook a 7 × 200-m incremental swimming step test on four occasions over an 8-month period before the 1998 Commonwealth Games (CG): January (10 d before the World Championships), May (early-season camp), July (midseason), and August (16 d before the CG). The lactate threshold (LT) was determined by a mathematical formula that calculated the threshold as a function of the slope and y-intercept of the lactate-velocity curve. Maximal 200-m test time declined initially from 127.7 ± 4.2 s (January 1998) to 130.2 ± 4.5 s (May 1998) and 129.1 ± 4.3 s (July 1998) before improving to 126.8 ± 4.2 s (August 1998) (P < 0.005). The swimming velocity at LT (s·100 m-1) also declined midseason before improving before the CG (P < 0.02) (January 1998: 70.5 ± 2.1; May 1998: 72.0 ± 2.2; July 1998: 72.2 ± 2.2; and August 1998: 70.8 ± 2.1). The blood lactate concentration at the LT decreased (P < 0.02) from 3.6 ± 0.2 mM to 3.2 ± 0.1 mM and 2.9 ± 0.2 mM before returning to 3.4 ± 0.2 mM for January, May, July, and August, respectively. The lactate tolerance rating (LT5-10), defined as the differential velocity between lactate concentrations of 5.0 and 10.0 mM, declined midway through the season (P < 0.015): 6.6 ± 0.5 s·100 m-1, 7.7 ± 0.5 s·100 m-1, 8.5 ± 0.5 s·100 m-1, and 6.9 ± 0.4 s·100 m-1, for January, May, July, and August, respectively. Despite these improvements in indicators of fitness, there was no significant improvement in competition performance across the season. Maximal effort 200-m time, lactate tolerance rating, and swimming velocity at LT (s·100 m-1) all improved in world-ranked swimmers with training, but these changes were not directly associated with competition performance.