Measurement of amounts or concentrations of catalysts is of great usefulness in science, technology, and especially in the health services. When it is impossible or impractical to measure amount of substance or mass of catalyst, its amount may be expressed by the catalysed change in a substrate entity as evidenced by some measure appropriate to the particular measuring system. Initially, the concomitant units were specific for each catalyst and measurement procedure. Later an independent universal unit was defined as "1 µmole of substrate utilized per minute under specified conditions of pH and temperature control". Eventually, catalytic activity (as a measure of amount) became defined by catalysed rate of conversion in mole per second with the suggested name "katal" (kat). A recent petition by the International Federation of Clinical Chemistry and Laboratory Medicine to the Comité International des Poids et Mesures, asking for inclusion of "katal" among the special names for SI derived units, was discussed and recommended by the Consultative Committee for Units and the Consultative Committee for Amount of Substance, and subsequently adopted by the 21st Conférence Générale des Poids et Mesures in 1999, referring to human health and safety and promotion of SI units. This decision should improve the traceability and comparability of results.