We measured the levels of thyroxine-binding prealbumin (TBPA), C-reactive protein (CRP), and interleukin-6 (IL-6) in sera from protein-energy undernourished and control children either without or with associated clinical infections. Levels of TBPA were significantly lower in undernourished than in control children without clinical infections. Mean serum TBPA concentration was also significantly lower in both clinically infected undernourished and control children than in those of the same nutritional status but without overt infections. Significant positive correlations were observed between the concentration of TBPA and the Z-scores of weight for age, height for age, and weight for height in all children without clinical infections, which disappeared in clinically infected children. CRP and IL-6 increased to approximately similar levels in sera from undernourished and control children with overt infections. An inverse correlation between the levels of TBPA and CRP and a significant positive correlation between the concentration of CRP and IL-6 were observed in children with associated clinical infections. Serum IL-6 and TBPA showed a small negative relationship in patients with overt infections. Thus, TBPA is a sensitive marker of undernutrition only in the absence of clinical infections, and the capacity to increase production of IL-6 and CRP during infections is preserved in protein-energy undernutrition.