Seventy-four patients with moderate to advanced periodontitis were classified by cigarette consumption at the initial exam: heavy smokers (HS) > or = 20 cigarettes/day (n = 31); light smokers (LS) < or = 19 cigarettes/day (n = 15); past smokers (PS) had a history of smoking but had quit by the initial exam (n = 10); and non-smokers (NS) had never smoked (n = 18). All patients were treated with four modalities of periodontal therapy followed by supportive periodontal treatment (SPT) for a period of up to 7 years. Clinical parameters including probing depth (PD), clinical attachment level (CAL), recession (REC), presence of bleeding on probing (BOP), and supragingival plaque (PL) were assessed at six sites around each tooth. Horizontal probing attachment level (HAL) was obtained at molar furcation sites. Data were collected initially, 4 weeks after non-surgical therapy, 10 weeks after surgical therapy, and yearly during SPT. HS and LS demonstrated less PD reduction and less CAL gain than PS and NS following active treatment and throughout SPT. Following active treatment, HAL changes were similar for all groups, but during 7 years of SPT, HS and LS experienced greater loss of HAL. There were no differences in BOP among the four groups. HS demonstrated a higher percentage of PL positive sites compared to the other groups. In summary, HS and LS responded less favorably to therapy than PS and NS. A past history of smoking was not deleterious to the response to therapy.