Medullasin is one of the serine proteases found in human bone marrow cells. The protease in granulocytes is assumed to play an important role in the induction of chronic inflammation. We measured the medullasin levels as a parameter of neutrophil function in granulocytes from patients with pustulosis palmaris et plantaris. All 17 patients studied underwent tonsillectomies, and the changes in the amount of medullasin were followed after surgery. Seven of the 17 patients were cured during the follow-up period (average: 21 months). In all the patients in whom PPP improved, the medullasin levels decreased significantly from the period of active disease when pustules were present to the inactive period. The amount of medullasin did not fall significantly in patients whose PPP did not improve. It is concluded that medullasin levels may reflect the activity of PPP and may serve as a criterion for performing a tonsillectomy.