BACKGROUND: Several studies have reported an association between smoking and periodontal destruction in young adults. AIMS: To investigate the effects of cigarette smoking on the periodontal condition of young Jordanian adults. METHODS: A case-control study of young adults between the ages of 20 and 35 years who were attending a dental hospital in Jordan for routine dental treatment. A group of 100 subjects who had smoked for at least 2 years were compared with 100 age- and sex-matched never smokers. Clinical measurements of plaque, bleeding on probing and probing depth were recorded and interproximal bone levels related to molar teeth were assessed from bitewing radiographs. RESULTS: The smokers had higher levels of plaque, more sites which bled in response to probing and a greater mean probing depth than never smokers. The unadjusted odds ratio for smoking to be associated with the presence of more than 10 pockets which were > or =4 mm was 3.08 (95% confidence interval 1.58-6.03). The proportion of measurable surfaces in smokers (22.7%) which had bone levels > or =3 mm below the cementoenamel junction was significantly higher (P<0.0001) than in never smokers (7.4%). Independent predictors of reduced bone levels in multivariate analysis were infrequent interdental cleaning (P=0.03), age of 30 or older (P=0.03) and smoking (P<0.0001). Within the multivariate analysis the adjusted odds ratio for smoking to be associated with reduced molar bone levels was 4.95 (confidence interval 2.48-9.88). CONCLUSIONS: It is concluded that cigarette smoking was a major environmental factor associated with accelerated periodontal destruction in the young adult Jordanians investigated.