OBJECTIVES: Assessment Towards Tobacco Economical and Medical Prospective Trial (ATTEMPT) is a prospective multinational observational longitudinal cohort designed to examine the natural course of successive smoking cessation attempts and their impact on health and economic outcomes. In order to evaluate the validity of the data collected via the Internet, self-reported weight and waist circumference (WC) were compared to in-home assessed measurements. METHODS: Subjects were recruited from existing Internet consumer panels Harris Interactive in 5 countries: Canada, France, Spain, the UK and the US. Subjects had to be aged 35–65 years, smoke at least 5 cigarettes per day, and intend to quit smoking. Assessments included questions on smoking status, health conditions, medical resource use and quality of life. Study participants were mailed at home standardized weight scales and tape measures with instructions. In-home assessments were performed by a health professional in a random sample of the US subjects right after they completed the quarterly Internet survey.RESULTS Outof the 4647 subjects included at baseline, 3242 (70%) completed the 3-month assessments and 2917 (63%) completed the 6-month assessments. In the US, 1147 (78%) subjects agreed to the in-home visit and 200 visits were conducted according to protocol specifications in December 2004 and January 2005. No statistically significant difference and high positive correlations were found between self-reported and observed weight (mean ±SD difference: under-self-reported +0.6±6.3 kg; correlation: 0.95) and waist circumference (over-self-reported -0.3±10.3 cm; 0.81). The distribution of demographic characteristics for the assessed sample was similar to characteristics of the remaining cohort. CONCLUSIONS: Based on good correlations between in-home visit and self-reporting on the web in the US, Internet is a reliable tool to collect health related data. As the ATTEMPT cohort progresses, this analysis will be reassessed with a greater sample size and further explored in other countries.