The incidence of elevated acute phase reactants, measured by nephelometry, was examined in 69 otherwise uncomplicated hemodialysis patients in comparison with 30 healthy subjects. Increased C-reactive protein was found in 40.6% of the patients (p less than 0.001 vs controls) and the degree of increase was correlated with the duration of hemodialysis. Haptoglobin was increased in 33.3% (p less than 0.01 vs controls). High levels of alpha-1-acid glycoprotein and alpha-1-proteinase inhibitor were present in 15.9% and 2.9%, respectively; these frequencies were not significantly different from controls. No differences between pre- and postdialysis values were observed. It is concluded that, unlike C-reactive protein, both alpha-1-acid glycoprotein and alpha-1-proteinase inhibitor maintain an excellent specificity in hemodialysis patients. Vice versa, haptoglobin may be unreliable as an acute phase reactant in these patients because of the unacceptably high false-positive rate.