The purpose of this controlled clinical trial was to compare the clinical efficacy of 3 treatment modalities in the treatment of deep interproximal intrabony defects. Thirty-six (36) defects in 36 patients were randomly assigned to 1 of 3 treatment groups by blocking to prognostic variables. The test was treated with bioresorbable membranes positioned coronal to the interproximal bone crest; the second group (membrane control) was treated with conventional non-resorbable (ePTFE) barrier membranes applied coronal to the alveolar crest; the third group (flap Control) was treated with an access flap procedure (MWF). No differences were observed in terms of baseline oral hygiene and defect characteristics among the 3 groups, indicating that the blocking approach was effective. A stringent infection control program was enforced for 1 year. The results indicated that: 1) at 1 year all treatment modalities resulted in clinically significant improvements in clinical attachment levels (CAL) and reductions in probing depths; 2) a statistically significant treatment effect (P < 0.0001, ANOVA) was observed comparing the test (4.6 +/- 1.2 mm), the membrane control (5.2 +/- 1.4 mm), and the flap control groups (2.3 +/- 0.8 mm) in terms of CAL gain; 3) differences in terms of CAL gain between the test (bioresorbable) and the membrane control (ePTFE) groups were not statistically significant (P = 0.19, t-test); 4) both the test and the membrane control groups gained significantly more CAL at 1 year than the MWF group (P < 0.0001, t-test). CAL gains > or = 4 mm were observed in 83.3% of cases in both GTR groups, while CAL gains of this magnitude were not detected in the MWF group. We concluded that clinically significant CAL gains can be obtained with GTR procedures using both bioresorbable and non-resorbable membranes. Patients' morbidity, however, was lower in the group treated with bioresorbable membranes.