Recently, transperineal interstitial-intracavitary applicators have been used to treat locally limited and advanced perineal and gynecologic malignancies. We have developed a single afterloading applicator, referred to as the "MUPIT" (Martinez Universal Perineal Interstitial Template), which with its prototypes has been utilized to treat 78 patients with malignancies of the cervix, vagina, female urethra, perineum, prostate, and anorectal region. The device basically consists of two acrylic cylinders, an acrylic template with a predrilled array of holes that serve as guides for trocars, and a cover plate. Some of the guide holes on the template are angled outward to permit a wide lateral coverage without danger of striking the ischium. The cylinders have an axial hole large enough to pass a central tandem or a suction tube for the drainage of secretions. Thus, the device allows for the interstitial placement of 192Ir ribbons as well as the intracavitary placement of either 137Cs tubes or 192Ir ribbons. In use, the cylinders are placed in the vagina and rectum and then fastened to the template, so that a fixed geometric relationship among the tumor volume, normal structures, and source placement is preserved throughout the course of the implantation. Hollow, closed-end, stainless steel trocars are then inserted through the guide holes that produce optimal coverage of the treatment volume. Appropriate computer programs also have been developed on a minicomputer for the corresponding dose-rate computations. These programs run with sufficient speed that they may be used for both the planning of the source placement beforehand and the computation of the actual dose-rate distribution obtained. The advantages of the system are (1) greater control of the placement of sources relative to the tumor volume and critical structures owing to the fixed geometry provided by the template and cylinders, and (2) improved dose-rate distributions obtained by means of computer-assisted optimization of the source placement and strength during the planning phase.