Introduction Mast cells (MCs) are widely distributed in connective tissues adjacent to vessels and nerves, and also beneath the epithelial surfaces [27]. MCs are best known for their role in inflammatory, hypersensitivity, and fibrotic disorders [20, 32]. MCs contribute to these processes by producing and secreting bioactive mediators. Recent studies have shown that MCs activate fibroblasts and promote collagen synthesis by producing and releasing fibrogenic substances. Hence, MCs play a role in the pathogenesis of chronic inflammation and fibrosis [31]. MCs can be divided into two subtypes based on dif- ferences in their neutral serine protease content. MC T contain only tryptase, whereas MC TC contain both tryptase and chymase in addition to other proteases [31]. MCs are the only cells known to contain tryptase [27]. Tryptase is a neutral protease of MCs and a good indicator of mast cell activation in allergic reactions [27, 29], and can induce the synthesis of type I collagen and can attract fibroblasts [1,18]. Immunostaining with anti- tryptase has been reported as being a very sensitive and specific method of detecting mast cells [15, 21, 23], and demonstrates significantly more cells compared to the histochemical methods [4, 22, 27]. Summary Recently, some studies reported the presence of mast cells in various malignancies and their role in tumor growth. The aim of the study was to determine the utility of mast cell numbers in evaluating benign and malig- nant prostate lesions, and to ascertain whether there are variations in the numbers of mast cells with the Gleason grade. The relationship between mast cell numbers and patient age was also investigated. Retrospectively, 104 prostate specimens were examined for the presence of mast cells. The study group consisted of 57 benign pro- static hyperplasias and 47 prostate carcinomas. The paraffin sections were stained with anti-human mast cell tryptase immunohistologically. The numbers of posi- tively staining cells in five high-power fields were counted, and their mean was calculated. There was no relationship found between mast cell numbers and age statistically. The mean mast cell numbers of the intratu- moral region were significantly different from those of the peritumoral region (p = 0.0001). While the differ- ence between benign hyperplasia and the intratumoral region was found to be significant (p = 0.0001), no dif- ference between hyperplasia and the peritumoral region was noted (p = 0.762). There was no statistical differ- ence between Gleason score groups (p = 0.452), and there was no interaction between score groups and intra- peritumoral regions (p = 0.355). Key words: Mast cells – Prostate Original Paper Immunohistological Analysis of Mast Cell Numbers in the Intratumoral and Peritumoral Regions of Prostate Carcinoma Compared to Benign Prostatic Hyperplasia Ozlem Aydin 1 , Duygu Dusmez 1 , Leyla Cinel 1 , Erdal Doruk 2 , Arzu Kanik 3 Mersin University Medical School, Departments of 1 Pathology, 2 Urology and 3 Biostatistics, Icel-Mersin, Turkey Pathol. Res. Pract. 198: 267–271 (2002) 0344-0338/02/198/4-267 $15.00/0 PATHOLOGY RESEARCH AND PRACTICE © Urban & Fischer Verlag http://www.urbanfischer.de/journals/prp Address for correspondence: Ozlem Aydin, Mersin Univer- sity Medical School, Department of Pathology, Eski Otogar Yani, 33070, Mersin-Icel, Turkey. Phone: +90-324/3374300; Fax: +90-324/3374305. E-mail: ozlemaydin66@hotmail.com