Akl et al. BMC Public Health 2011, 11:244 http://www.biomedcentral.com/1471-2458/11/244 instrument reported (n = 7); self developed validated instrument (n = 1); previously developed instrument with no validation reported (n = 1); previously validated instrument that measured “forms of smoked tobacco products other than cigarettes” which the authors assumed to be waterpipe smoking (n = 1) and not reported (n = 13). Fifteen studies reported pilot testing the measurement instrument. Twenty-nine studies reported response rate that varied from 18% to 62% for online administered surveys, 46% to 100% for paper- based surveys, and 70% for telephone survey. Additional file 3 provides detailed description of the characteristics of included studies by world region. Table 1 provides a summary description of these studies. Figures 3, 4 and 5 respectively present the prevalence for school students, university students and adults across countries for which published studies were iden- tified. In cases where more than one study was available, we used medians. In most of these cases, the values relate to specific populations in that country and not to the general population. The following paragraphs of the results section provide synthesized information by region. Middle East region School students two studies of intermediate and secondary school stu- dents in Lebanon reported 25% current and 65%-66% ever waterpipe smoking (Additional file 3; Part 1) [16-18]. Waterpipe smoking was associated with male sex, [16,17] attending public schools, and secondary classes [16,18]. The prevalence of current waterpipe smoking was about 10 times that of current cigarette smoking [16-18]. Two studies of secondary school boys in Saudi Arabia reported 9% and 10% current waterpipe smoking respectively [19,20]. A study of secondary school boys in Bahrain reported 13% waterpipe smoking and 2% waterpipe only smoking [21]. A multi-national study conducted in all schools with grades of 13-15 year old students reported current waterpipe smoking preva- lence of 15% in Bahrain, 16% in Kuwait, 9% in Oman, 14% in Qatar, 15% in United Arab Emirates (UAE), and 15% in Yemen [20]. The study reported that boys were significantly more likely than girls to use waterpipe in all countries except Qatar. Waterpipe smoking was sig- nificantly more common than cigarette smoking among boys and girls in the majority of countries included in this survey. University students one study from Lebanon reported 28% current water- pipe smoking and 43% ever waterpipe smoking, [22] while another study reported 21% waterpipe only smok- ing [23]. Waterpipe only smoking was associated with male sex, and alcohol use (> 7 pints beer/day) [23]. The Page 4 of 12 prevalence of waterpipe only smoking was about 3 times that of cigarette smoking only (Additional file 3; Part 1) [22,23]. Two studies reported 8% ever waterpipe smok- ing among male medical students of Saudi Arabia and 6% current waterpipe smoking among students of Uni- versity of Sharjah in UAE, respectively [24,25]. Com- pared with cigarette smoking, the prevalence of waterpipe smoking was slightly higher in Saudi Arabia and slightly lower in the United Arab Emirates. One study from Syria reported 15% current and 46% ever waterpipe smoking [9,26-28]. Waterpipe smoking was associated with male sex, cigarette smoking, number of friends smoking waterpipe only, number of friends smoking both cigarette and waterpipe, and number of waterpipes smoked daily in the house. The prevalence of cigarette smoking was slightly higher than that of water- pipe smoking. One study of students of secondary schools in villages of Egypt reported 19% ever waterpipe smoking [29]. Three studies conducted among male adults residing in villages reported 11%-15% prevalence of waterpipe smoking in groups [30-32]. Adults one study from Lebanon found 15% current waterpipe smoking and 5% group waterpipe smoking [33]. One study of adults attending primary care and specialty clinics found 4% waterpipe only smoking. One study of school students found a prevalence of 9% waterpipe only smoking by their parents at home [23]. The preva- lence of cigarette only smoking was about 5 times that of waterpipe only smoking (Additional file 3; Part 1). A study of government ministry employees in Kuwait found 63% ever waterpipe smoking [34]. A study of female teaching staff and employees of King AbdulAziz University of Saudi Arabia reported 11% current water- pipe smoking with moderate correlation between water- pipe smoking and cigarette smoking [35]. A study of school teachers in Saudi Arabia reported 4% current waterpipe smoking [19]. A study of adults aged 19 and above in Bahrain reported 5% current waterpipe smok- ing. A study of physicians in Bahrain and Kuwait reported 6% and 12% current waterpipe smoking, respectively [36]. The prevalence of cigarette smoking was 1-3 times that of waterpipe smoking. One study from Syria found a prevalence of 12% of current water- pipe use [37] while another study found a prevalence of 9% of current waterpipe smoking [38]. The prevalence of cigarette smoking was 3-4 times that of waterpipe smoking [37,38]. Pregnant women 3 studies reported 7%-18% ever smoking and 5%-6% waterpipe smoking during pregnancy [39-41]. The pre- valence of cigarette smoking was about 3-5 times that of waterpipe smoking during pregnancy and about 3 folds before pregnancy (Additional file 3; Part 1).