This study was conducted to determine whether fixed partial dentures supported by dental implants provide an acceptable alternative to conventional removable partial dentures in patients with Kennedy class I or class II edentulous conditions. The acceptability of the new treatment will be based on success rates, impact on the health of the remaining dentition, masticatory performance, patient satisfaction, and maintenance care and cost. The study was planned also to provide comparisons between two designs commonly used by dentists for fabricating removable partial dentures. The designs differed only in terms of the type of the retainer (clasp type) and tooth support (rest location). A total of 272 patients with Kennedy class I and class II edentulous conditions were assigned on a random basis to one of the treatment groups, 134 to receive a removable partial denture and 138 a fixed partial denture supported by a blade-vent implant. All of the patients were medically screened and met prespecified criteria for oral hygiene, bone support for abutment teeth, and size of the residual ridge. Thirty-four patients were eliminated from the study before completion of their treatment. An additional six patients with early implant failures were reentered in the study and followed up as a separate group. The remaining 232 patients received comprehensive dental care, including removable partial dentures for 118 and fixed partial dentures for 114 patients. A series of examinations, radiographs, masticatory performance tests, patient satisfaction, food selection questionnaires, and dietary history were completed before initiation of the treatment, 16 weeks after the insertion of an RPD or an implant, and thereafter at 6-, 18-, 36-, and 60-month intervals. In addition, patients were seen at 6-month intervals for a recall dental examination, oral prophylaxis, plaque instructions, radiographic survey of the implant, and any needed dental treatment. The randomization stratification approach produced two treatment groups with comparable age, number of remaining maxillary and mandibular teeth, type of opposing dentition, and percent of patients with Kennedy class I and class II edentulous conditions. The mean scores of bone support, tooth mobility, and sulcular depths of abutment teeth were also similar. Significant but comparable improvements in oral hygiene and sulcular depth occurred in the two groups after treatment. The periodontal health scores at the 16-week interval serve as the baseline to measure subsequent changes in periodontal health.(ABSTRACT TRUNCATED AT 400 WORDS)