We studied the clinical effectiveness of oral hygiene instruction given to 71 "high" plaque patients in 19 Washington State dental practices to determine the extent of plaque reduction. Patients were examined prior to instruction and prophylaxis. Therapist instructions were tape-recorded and content analyzed, and therapists' expectations were scored. Patients were surveyed as to oral health behaviors and general health habit characteristics. Only 20 of 71 (28%) initially "high" plaque patients had reduced mean plaque scores to 1.0 or less at six weeks. An additional 13 percent had deteriorated plaque levels at 24 weeks. We concluded that therapists did not consistently follow principles of effective instruction and often overestimated their effectiveness. Determinants of relapse involve the educational process, the patients' oral status, and patients' life situations.