Higher body mass index is associated with a higher incidence of colorectal cancer (CRC) but also with improved survival in metastatic CRC (mCRC). Whether weight change after mCRC diagnosis is associated with survival remains largely unknown.We analysed individual patient data for previously untreated patients enrolled in five phase 3 randomised trials conducted between 1998 and 2006. Weight measurements were prospectively collected at baseline and up to 59.4 months after diagnosis. We used stratified multivariable Cox models to assess the prognostic associations of weight loss with overall and progression-free survival, adjusting for other factors. The primary end-point was a difference in overall survival (OS) between populations with weight loss and stable or increasing weight.Data were available for 3504 patients. The median weight change at 3 months was -0.54% (IQR -3.9 +1.5%). We identified a linear trend of increasing risk of death associated with progressive weight loss. Unstratified median OS was 20.5, 18.0, and 11.9 months (p