Background: Obesity is associated with poorer outcomes across multiple cancer types. Health behaviour change (smoking cessation, physical activity (PA) and alcohol moderation) can improve both obesity and outcomes among cancer survivors. Methods: Cancer patients (pts) were cross-sectionally surveyed on their smoking, PA and alcohol use before and after diagnosis and their perceptions of these behaviours on quality of life (QoL), fatigue, survival (OS) and safety. Multivariable logistic regression models evaluated the association of BMI 1 year prior to diagnosis with behaviour changes and perceptions. Results: Of 1269 pts, 204 smoked at diagnosis and 58% quit afterwards; 350 met PA guidelines at diagnosis and 238 at follow-up; 661 drank alcohol at diagnosis and 50% reduced consumption afterwards. Median BMI was 25.8 (22% obese). Most pts perceived PA ( > 75%) as improving outcomes, smoking ( > 70%) as worsening outcomes and half (41-49%) felt alcohol worsened outcomes. At diagnosis, increased BMI was associated with being an ex-smoker (vs current smoker; P= 0.003), never using alcohol (vs former use; P= 0.05) and not meeting PA guidelines (P= 0.01). Among smokers at diagnosis, increased BMI was associated with smoking cessation (aOR 1.08 per 1 unit BMI increase, P= 0.03) and perceiving that smoking worsens OS (aOR 1.10, P= 0.04) and fatigue (aOR 1.08, P= 0.08). Among pts not meeting PA guidelines at diagnosis, increased BMI was associated with perceiving that PA worsens fatigue (OR 1.02, P= 0.06) and is unsafe (OR 1.04, P= 0.06). Among drinkers at diagnosis, increasing BMI was associated with perceiving alcohol to be less harmful (aOR 0.93, P= 0.002), less likely to worsen OS (aOR 0.96, P= 0.04) and fatigue (aOR 0.97, P= 0.09). BMI was not associated with changes in alcohol or PA after diagnosis. BMI was not associated with counselling rates; however, 66% of current smokers received cessation counselling while only 14% of current drinkers and 13% of pts not meeting PA guidelines received counselling on their respective behaviours. Conclusions: Obese pts were more likely to quit smoking and perceive it to be harmful but less likely to perceive alcohol as harmful. Survivorship programs should consider focusing on PA and alcohol counselling in obese pts.