The aim of the present study was to evaluate a method for calculating arterial values of pH, carbon dioxide tension (P(CO(2))) and oxygen tension (P(O(2))) from peripheral venous values. In total, 40 patients were studied. Arterial and peripheral venous blood were sampled at a department of respiratory diseases. Arterial values were calculated from venous, and measured and calculated values of arterial pH, P(CO(2)) and P(O(2)) were compared. Measured and calculated values of pH and P(CO(2)) correlated well, with the difference between them having a very small bias and standard deviation (pH -0.001+/-0.013, P(CO(2)) -0.09+/-0.28 kPa) within those considered acceptable for laboratory equipment and clinical practice. All but four patients had peripheral oxygen saturation (S(p,O(2))) <or=96%, and for these measured and calculated P(O(2)) correlated well, with a difference such that the bias and standard deviation suggested that calculated P(O(2)) may be clinically useful (P(O(2)) 0.11+/-0.53 kPa). The present study evaluates a method for calculating arterial pH, carbon dioxide tension and oxygen tension from venous blood. It has been shown that arterial pH and carbon dioxide tension can be calculated precisely, and that oxygen tension can be calculated with reasonable precision in the vast majority of patients. This method might be useful in reducing the need for painful arterial punctures.