OBJECTIVES: High Body Mass Index (BMI) and obesity, prevalent throughout industrialised societies, are known to be associated with many co-morbidities. A possible increase in upper respiratory tract infections (URTI) associated with higher BMI was investigated in this study. METHODS: Cohort study using prospectively recorded patient data within the Full Feature General Practice Research Database (GPRD) which represents approximately 5% of UK population. Study subjects were categorised according to their baseline BMI into five exposure groups: I 18.5 – 24.9; II 25.0 – 29.9; III 30.0 – 34.9; IV 35.0 – 39.9 and V 40.0 and above. BMI records of 50.0 and over were discarded. Patients were followed from January 1, 1998 until December 31, 2002. Study outcomes were either Ear Nose and Throat (ENT) infections, or Respiratory Tract Infections (RTI). Statistical analysis : For each outcome group, crude incidence rates and incidence rate ratios (IRR) by BMI category were estimated. Poison regression analysis was used to make adjustments for age, gender, asthma, COPD, diabetes, smoking status, GP consultations and sleep apnoea. RESULTS: A total of 244,479 patients were eligible for the cohort. The adjusted IRR for ENT infections and URTI respectively was for BMI group II, 1.13 (1.11,1.15) and 1.10 (1.08, 1.12), for BMI group III, 1.18 (1.15,1.21) and 1.21 (1.17,1.24) BMI group IV, 1.23 (1.17,1.28) and 1.33 (1.27,1.39) and for BMI group V 1.29 (1.20,1.38) and 1.42 (1.32,1.53). These findings were consistent after stratification by the number of GP consultations in 1997. CONCLUSIONS: This study provides evidence that there is an association between increasing BMI and ENT and upper respiratory tract infection. This association should be taken into consideration in the evaluation of the burden of obesity.