To study the prevalence of diabetes mellitus (DM) and impaired glucose tolerance (IGT) and their risk factors in Jordan. A cross-sectional study in four Jordanian communities was conducted: Sarih in the north, Sikhra in the middle of the country, Mazar in the south and Subha-Subheih in the east. Within each community, a systematic sample of households was selected. All subjects > or =25 years of age within the selected households were invited to participate in the study. A total of 2836 subjects were actually included in the study with an overall response rate of 70.5%(45% in males and 86% in females). World Health Organization criteria were adopted for the diagnosis of DM (fasting plasma glucose > or =7.8 mmol L(-1) or plasma glucose of > or =11.1 mmol L(-1), 2 h after an oral anhydrous glucose load of 75 g) and IGT (fasting plasma glucose <7.8 mmol L(-1)and plasma glucose between 7.8 mmol L(-1) and 11.1 mmol L(-1), 2 h after an oral glucose load of 75 g). The overall prevalence of DM was 13.4%: 14.9% in males and 12.5% in females. IGT was found in 9.8% of the study population: 9.0% in males and 10.3%, in females. Factors independently related to DM using stepwise logistic regression analysis were sex, age, family history of DM, hypertension, hypercholesterolaemia (HC), and hypertriglyceridaemia (HTG). Diabetes mellitus and IGT are common among adult Jordanians. Considering the high prevalence of this sickness makes it imperative to formulate a national plan to face this disease and its complications.