A number of clinically important conditions is associated with insulin resistance and compensatory hyperinsulinemia. Since there is growing evidence that serum insulin measurement can be useful for the individual characterization of the metabolic situation, we aimed to study the agreement of serum insulin results obtained with different widely used immunoassay systems. Comparative measurement of 100 serum samples was performed with three systems (Abbott AxSYM; Bayer Centaur; and Roche Elecsys). Close correlation of the results was found with Kendall's Tau > 0.95 for all three comparisons. Between the Abbott and the Bayer assay, and between the Abbott and the Roche assay, a slope of approximately 10% was observed. Mean concentrations found were 29.2 microU/mL, 30.0 microU/mL, 27.1 microU/mL with Abbott, Bayer, and Roche, respectively. It is concluded that the three assay systems studied offer a good agreement of results with only minor systematic bias.