This study investigatesthe effects of the placement of a bioresorbable Type I collagen barrier on clinically healthy sites. Ten human subjects, with at least one pair of matched periodontal defects included in a previous study, were investigated for the effect of guided tissue regeneration (GTR) membrane placement on adjacent healthy sites. Control sites consisted of open flap debridement, while experimental sites consisted of similar debridement procedures with placement of a collagen membrane for GTR. The membrane placement was designed to completely cover the periodontal defect, and therefore of necessity overlapped adjacent periodontally healthy sites. It is these healthy sites that are included in the present study. Standardized measurements of change in probing attachment levels were obtained at the time of surgery and 1 year later. The differences in change of clinical attachment levels (CAL) were compared utilizing the Student r‐test for paired samples. The mean loss of CAL in the control sites was 1.00±1.179 mm, and in the test sites was 0.60±1.729 mm. When comparing the difference in changes of CAL in both test and control sites, there was no statistical significance. There was, however, a wide range of changes between individual sites, which might suggest a clinically meaningful change of CAL on an individual tooth basis. The results of this study suggest that the placement of a bioresorbable collagen barrier has no significant effect on CAL in healthy areas.J Periodontol 1993; 64:57–59.