Hypertrophic scars and keloids appear to be unique to humans since animals are not known to form these lesions. Therefore, in an effort to develop an experimental model for their study, implants of these human lesions were made in nude (athymic) mice (nu/nu) in suprascapular subcutaneous pockets. The implants were recovered from 2 to 246 days. By histological and fine structural parameters all implants remained viable and their morphological character was maintained. Selected mice were injected with barium to confirm by microangiography vascular flow between mouse and implant. Hoechst stain for DNA, used to distinguish mouse cells from human cells, confirmed vascular anastamosis between host and implant: barium-filled vessels in the interior of the implant demonstrated human endothelial cells. Peripheral vascularization of the implant with minimal ingrowth of mouse vessels occurs during the first 8 days. Anastamosis probably occurs sometime before 16 days postimplantation, or earlier, depending upon the availability of patent microvessels in the implanted tissue. The presence of the implant does not appear to prompt a continuing vascular growth into or throughout the implant. The time frame of 16 days postimplantation should be taken into account when developing schemata of experimental or therapeutic modalities.