The relation between smoking and periodontal disease as classified by the Community Periodontal Index of Treatment Needs (CPITN) code was investigated in 1, 611 Japanese male factory workers in Osaka, 20-59 years of age. The Simplified Debris Index (DI-S) according to the Simplified Oral Hygiene Index was employed as an index for the quantity of dental plaque. For the items related to smoking, a self-reporting form on smoking, the number of years of smoking, and the number of cigarettes smoked was distributed prior to the oral examination. Our results indicated the following: in the group with a poor state of oral hygiene, the negative effects of smoking were evident, resulting in unhealthy periodontal tissue. We believe that this result further confirms the contention that the removal of dental plaque is most important as a prophylaxis for periodontal diseases, and public education about this negative consequertce of smoking will further strengthen prophylactic measures.