The purpose of this study was to determine the influence of furcation involvement (FI) and mobility (MO) on change in probing periodontal attachment level (AL) on molar teeth. Twenty-four patients were selected from The University of Michigan longitudinal periodontal clinical trial. The patients who met the entry criteria received a baseline examination and were treated by periodontal scaling and root planing followed by different periodontal surgical approaches in a split-mouth design. Patients were placed on a 3-month interval for maintenance prophylaxis and had yearly periodontal scorings for a period of 8 years. The scorings included determination of AL, FI, MO, and tooth loss (TL). Data for molar teeth from three different time periods (at entry (A) and at 1 (B) and 8 (C) years) were analyzed. The results demonstrated that teeth with FI were 2.54 times more likely to be lost when compared to teeth without FI during the maintenance period. By using paired t test approach to the probing attachment level data, molars with FI had significantly more loss between times B and C but not between A and B or A and C. Teeth with increased MO demonstrated significantly more AL between times A and C and B and C. When the interaction between MO and FI was analyzed, teeth with both FI and MO had significantly more attachment loss during the maintenance period (B to C). No statistically significant difference was found among the three different therapeutic approaches (pocket elimination, curettage, and modified Widman flap) during any of the time periods.(ABSTRACT TRUNCATED AT 250 WORDS)