AbstractA study was undertaken in order to test the effect of hygiene instructions given by specially trained chair‐side assistants and to test the value of depuration prior to instructions. Chair‐side assistants were trained to motivate and instruct patients in oral hygiene methods. The 71 participants (12 female and 59 male employees of the same industrial firm) were divided into three groups. Group C received depuration (one visit) prior to oral hygiene instructions, Group B hygiene instructions only, and Group A depuration only. Following instructions, the participants were examined after 4, 12 and 24 weeks with respect to: DMFT, Gingival Index, Retention Index, plaque surface and periodontal pockets ≧ 4 mm. The results showed a reduction in plaque surfaces from baseline to 24 weeks in Groups B and C of 67% and 70% (difference not significant). Slight improvement was noted in Group A. Significant improvement was also observed in Gingival Index scores and pocket depths. Significant differences were not found between the groups with respect to Retention Index scores. Smoking did not influence the program. The effects of instructions were retained after 6 months. It is concluded that the use of this kind of personnel is highly effective and expedient, and that the present model can be useful in preventive dental health