The purpose of this study was to compare tooth mobility at different time periods during periodontal treatment and to relate changes in mobility to each method of treatment. Ninety-three patients (2421 teeth) with moderate to severe periodontitis were scored: (1) at initial examination, (2) 1 month following scaling, root planing, instruction in oral hygiene and preliminary occlusal adjustment, (3) 1 month after treatment with pocket elimination or reduction, curettage, modified Widman flap or scaling and root planing by the periodontist, and (4) 1 and 2 years following completed periodontal treatment. After initial nonsurgical treatment there was a significant decrease in tooth mobility. Tooth mobility increased temporarily after pocket reduction surgery but was not altered following curettage, modified Widman flap or scaling and root planing. Two years post-treatment there was a trend toward further decrease in tooth mobility with professional tooth cleaning every 3 months.