Introduction Surgery is a major risk factor for venous thromboembolism. The incidence of venous thromboembolism is also known to increase with increasing body mass index (BMI). We describe the risk of hospital admission for (or death from) venous thromboembolism in relation to BMI, both in the absence of surgery and in the 12 weeks after surgery.

Methods Women in the UK were recruited into the Million Women Study in 1996–2001 and followed by record linkage to routinely collected inpatient and day case NHS hospital admissions and deaths. The incidence of hospital admission or death for venous thromboembolism was estimated in relation to BMI both with and without surgery.

Results 1.2 million women with an average BMI of 26.2 kg/m² were included in these analyses, 55% of whom had at least one hospital admission for surgery during follow-up. The RR of venous thromboembolism increased with increasing BMI both with and without surgery. In the absence of surgery those of BMI ≥35 kg/m² were 3.5 times more likely to be diagnosed with venous thromboembolism than those of BMI 22.5–24.9 kg/m² [RR 3.5 (95%CI 3.1 to 3.9)]. A similar relationship was observed in the 12 weeks following day or inpatient surgery, but the absolute risks associated with BMI were much greater following surgery.

Conclusions The risk of venous thromboembolism increases with increasing BMI and the associated excess risk is much greater in the 12 weeks following surgery than without surgery.