Background and Objective: Diabetes Mellitus is a metabolic disease and the most prevalent disorder in pregnancy. Gestational diabetes mellitus is a kind of diabetes that is recognized in pregnancy. Many risk factors have been recognized for gestational diabetes mellitus. Determining new risk factors help to identify women who are at risk for diabetes. This research was conducted in order to determine the relationship between cigarette smoking before or during pregnancy and gestational diabetes mellitus in women referring to health care centers in Tehran during 2008-2009. Materials and Methods: This case –control study was conducted on 110 pregnant women with gestational diabetes (case group) and 96 pregnant women without the condition (control group) referring to healthcare clinics affiliated to Research Centers of Endocrinology and Metabolism at Ayatollah Taleghani Hospitals as well as Iran Research Institute of Endocrinology and Metabolism. A questionnaire was used for collecting data by interviewing the subjects. Both groups were matched for age and parity. Results: Demographic characteristics were similar in both groups. There was a significant difference between the two groups in cigarette smoking before or during pregnancy and occurring diabetes mellitus (p=0.0001) with an estimated odds ratio (OR) of 3.79 (%95 CI = 1.37 – 10.53). Conclusion: The present findings showed a significant relationship between cigarette smoking and gestational diabetes mellitus.