Routine results, pH, partial pressure of carbon dioxide (pCO2) and of oxygen (pO2), and standard hydrogen carbonate ion concentration (SBC) in identical specimens of arterial blood from patients deviate substantially. The results from seven laboratories (each laboratory examining the same 12 patients and the same five types of quality control materials) and evaluation in terms of accuracy and precision suggest the variations between days (delta 2) and single measurements (sigma 2) to be the main factors for these deviations. A reduction of these variations must have the highest priority in quality control programmes, since the variations mask possible true level deviations between laboratories. The five control materials (Qualicheck, Quantra whole blood level I-II-III and hemolyzed donor blood) are not fully optimal as substitutes for patient blood in such quality control programmes.