Summary A high content of mast cells (MC) is considered characteristic of neurofibromas but not of malignant schwannomas and neurilemomas. We examined the extent and reliability of this finding by counting MC in 61 peripheral nerve sheath tumors and in 103 non-neurogenic soft tissue sarcomas. We furthermore investigated correlations between the amount of MC and various features of the tumors (e.g. grades of malignancy). Neurofibromas had very high mast cell counts. However, this result only applied to about 70'% of these tumors. Malignant schwannomas, malignant fibrous histiocytomas and leiomyosarcomas had remarkably high median values of MC counts with a wide dispersion within the histological groups. Synovial sarcomas were the only group that contained MC in every case, though often in small numbers. In univariate analyses the number of MC was negatively correlated to grades of malignancy, cellularity and mitotic activity of the sarcomas and tended to correlate positively to the amount of myxoid and collagenous connective tissue and lymphocytic infiltrates. Multiple linear regression analysis revealed a significant correlation to the grade of malignancy and the amount of connective tissue.