BACKGROUND/AIM: In Switzerland monitoring of obesity in the general population is based on body mass index (BMI) derived from self-reported weight and height. This approach may lead to misclassification of obese subjects and misinterpretation of obesity prevalence and trends. In order to explore this potential bias, we compared studies with measured and self-reported data. METHODS: We analysed five studies based on measured BMI and five studies based on self-reported BMI, all of which were carried out in Switzerland between 1977 and 2004 and encompassed men and women aged 35-74 years. Obesity was defined as BMI>or=30 kg/m2. RESULTS: The prevalence of obesity was markedly higher (1.6 times) in studies with measured BMI in both sexes: 14.2% vs 8.8% in men and 12.5% vs 7.9% in women. These differences tended to increase with age in both sexes. However, a similar upward trend in the prevalence of obesity was observed with both methods (absolute increase per year in men and women respectively: 0.24% and 0.25% using measured BMI vs 0.17% and 0.20% using self-reported BMI). CONCLUSION: In Switzerland obesity prevalence in adults has clearly increased in the past three decades. Although the use of self-reported height and weight leads to a valid estimation of this increase, it results in a considerable underestimation of obesity prevalence rates in Switzerland. The type of assessment of height and weight should be taken into consideration when comparing prevalences of obesity between studies or regions or when using these prevalences to assess associated health risks or costs.