The purposes of this study were to determine the effects of 4 wk of intensified training on resting plasma glutamine concentration, and to determine whether changes in plasma glutamine concentration relate to the appearance of upper respiratory tract infection (URTI) in swimmers during intensified training. Resting plasma glutamine concentration was measured by high performance liquid chromatography in 24 elite swimmers (8 male, 16 female, ages 15-26 yr) during 4 wk of intensified training (increased volume). Symptoms of overtraining syndrome (OT) were identified in eight swimmers (2 male, 6 female) based on decrements in swim performance and persistent high fatigue ratings; non-overtrained subjects were considered well-trained (WT). Ten of 24 swimmers (42%, 1 OT and 9 WT) exhibited URTI during the study. Plasma glutamine concentration increased significantly (P = 0.04, ANOVA) over the 4 wk, but the increase was significant only in WT swimmers (P < 0.05, post-hoc analysis). Compared with WT, plasma glutamine was significantly lower in OT at the mid-way timepoint only (P < 0.025, t-test with Bonferroni correction). There was no significant difference in glutamine levels between athletes who developed URTI and those who did not. These data suggest that plasma glutamine levels may not necessarily decrease during periods of intensified training, and that the appearance of URTI is not related to changes in plasma glutamine concentration in overtrained swimmers.