This case-control study compares the prevalence of cigarette smoking among patients in a periodontal practice (cases) with that of patients in referring general dental practices (controls). Smoking histories of patients (age greater than or equal to 25 years) in a periodontal practice and five general dental practices were obtained by questionnaire. From the general practices, only patients reporting negative histories for periodontitis were studied. Periodontal status of the periodontal practice patients was based on bone loss from full mouth radiographic surveys and gingival pocket depths. Patients were stratified by age (25 to 40, 41 to 55, and greater than 55 years) and sex. The combined frequency of current or former cigarette smoking reported by 196 periodontal practice patients with moderate or advanced periodontitis (M-A perio group) was higher than that reported by 209 general dental practice patients (gen prac group) in all age and sex categories. The age and sex adjusted summary odds ratio for a positive smoking history among M-A perio subjects relative to gen prac subjects was 2.6 (P less than 0.001). Separate corresponding odds ratios (age and sex adjusted) for current smoking versus never smoking in the two groups were 3.3 (P less than 0.001) and for former smoking versus never smoking 2.1 (P less than 0.004). Among current smokers, patients in the M-A perio group reported heavy smoking (greater than 10 cigarettes/day) relatively more often than control subjects (adjusted R.O. = 5.7; P less than 0.001). In the M-A perio group the frequency of current smoking increased with disease severity.(ABSTRACT TRUNCATED AT 250 WORDS)