The change from measuring enzyme catalytic activity concentrations from 25 degrees C to 37 degrees C in the German Federal Republic has led to the need for new reference ranges for defined patient groups and for healthy individuals. Up to now, these are only present as tentative values and are incomplete, especially for children. This article describes a method for deriving reference ranges from results obtained from measurement at 25 degrees C and 37 degrees C and the use of percentiles to establish values for 37 degrees C. A total of 1,111,378 data from 507,305 patients were used to establish reference ranges for the following 11 enzymes at 37 degrees C using the test kits from Roche Diagnostics measured on the Modular analyser: acid phosphatase, alkaline phosphatase, alanine aminotransferase, aspartate aminotransferase, cholinesterase, creatine kinase, creatine kinase - MB subunit, gamma glutamyl transpeptidase, glutamate dehydrogenase, lactate dehydrogenase and lactate dehydrogenase - isoenzyme 1. The computed reference ranges from the data used gave rise to reference ranges, some of which were in agreement with the data from the producer, some of which, however, showed deviations from the values given by the producer. Ranges for newborns, children and adolescents could be computed with the prerequisite that ranges for 25 degrees C were available and that these had been established and validated. This method of establishing reference ranges for catalytic enzyme activities can be used for all producers, providing the number of data used is sufficient to allow for valid statistical analysis.