Several well tolerated materials are currently available for the treatment of hernias. Polyester (Dacron) and polypropylene (PPL) are currently the most frequently used materials. Experimental work has highlighted the biological response of laboratory animals treated with a range of prostheses. Authors agree that an inflammatory response rarely occurs, although materials generate a range of responses. In the years 1996–2000, hystological tests were performed to assess thein vivo response to the range of materials. Thirty-five prostheses were examined in 34 patients. Twenty-six were men and 8 were women with an average age of 59.3 years (range 35 to 89). Samples were fixed in formalin, de-hydrated and placed in paraffin. Four micron sections were dyed with hematoxylin and eosin and then underwent microscopy. The type and the amount of prosthesis-englobin connective tissue was evaluated, as was the number of giant cells, of inflammatory elements and of vessels. Evaluation scores ranged from-to ++. The microscope highlighted a range of patterns according to the implant: a) Dacron prostheses were surrounded by a large amount of thick connective tissue, with scarce fibroblastic elements, engendering a lively response with polynucleate giant cells in relatively large quantities, b) PPL plug prostheses were englobed in a looser connective tissue, rich in fibroblasts and with a scarce amount of inflammatory elements and giant cells, c) PPL patches were surrounded by thicker connective tissue, appear to be more inert, and were often surrounded by adipose tissue. The type of prostheses, as well as its form, may engender different tissue responses.