Structural and ultrastructural tissue modifications caused by polytetrafluoroethylene (teflon) paste injection in incontinent male patients have been studied. The most considerable observations are: a diffuse interstitial splitting of teflon paste in particles of about 3-100 microns. Given that the tissue reaction is limited to a modest infiltration of lymphocytes and monocytes and a slight increase in the collagen fibres, it is concluded that the teflon paste injection is well tolerated by the tissues.