Keyes' method of non-surgical therapy was compared with modified Widman flap surgery in 9 patients with symmetrical periodontal disease. Following an initial oral hygiene programme, baseline measurements were recorded and paired contralateral areas were subjected randomly to the 2 techniques. 42 teeth receiving surgery were compared with 40 treated by Keyes' method. 6 sites per tooth were scored immediately prior to therapy and 3 months later, using a constant force probe with onlays. Consistent data were recorded for the 6 separate sites, which showed no baseline difference between treatments, slightly greater recession with surgery at 3 months, but no difference between treatments in probing depth and attachment levels. Mean data for individual patients showed similar consistency. Probing depth in deep sites was reduced slightly more with surgery, and there were no differences in bleeding on probing at 3 months. Both techniques gave marked improvements in health. Surprisingly, only 2 subjects preferred Keyes' technique of mechanical therapy, 6 preferred surgery, and 1 had no preference.