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Objective: Obesity increases risk of several cancers, but it is uncertain whether weight reduction is followed by any decreased risk. To address this topic, we selected a group of patients representing a substantial weight loss starting at a defined time, i.e. patients submitted to obesity surgery. We hypothesized that risk of obesity-related cancer decreases with time after obesity surgery.Design and setting: A nationwide, population-based cohort study of obesity surgery in 1980-2006 registered in the Swedish Patient Register. New cancers were identified through the Swedish Cancer Register. Cohort members’ observed total number of overall obesity-related cancers and groups of obesity-related cancer (breast, prostate, colorectal, endometrial, kidney) were divided by the expected numbers, representing the baseline risk, thus calculating standardized incidence ratios (SIRs) with 95% confidence intervals (CIs). Time trends of SIR after obesity surgery were the main outcome measure. Results: Among a total of 13,123 obesity surgery patients, contributing with 125,049 person-years of follow-up, 296 new cases of obesity-related cancer were identified. There was no overall decrease in SIR of obesity-related cancer with increased time after obesity surgery (p for trend 0.40). Similarly, no statistically significant trends with follow-up time were found for cancer of the breast (p=0.60), prostate (p=0.34), endometrium (p=0.83), or kidney (p=0.42), while the risk of colorectal cancer increased with time (p for trend 0.01) after obesity surgery. Conclusions: The weight reduction following obesity surgery might not be entailed by a decreased risk of obesity-related cancer with increasing follow-up time as compared to the baseline risk.
Relative cancer risk, SIR (95% CI)

Years after surgery	All	Breast cancer	Prostate cancer	Colorectal cancer	Endometrial cancer	Kidney cancer
1-4	1.0 (0.7- 1.2)	0.5 (0.3- 0.8)	1.2 (0.4- 2.9)	0.7 (0.1- 1.9)	2.1 (1.0- 3.7)	2.9 (1.1-6.3)
5-9	1.0 (0.8- 1.2)	0.4 (0.3- 0.7)	0.8 (0.3- 1.7)	1.4 (0.7- 2.6)	2.3 (1.4- 3.7)	2.5 (1.0- 5.2)
≥10	1.1 (0.9- 1.3)	0.6 (0.4- 0.8)	0.7 (0.3- 1.3)	2.1 (1.3- 3.2)	2.1 (1.4-3.2)	3.7 (1.5- 5.5)
P for trend	0.40	0.60	0.34	0.01	0.83	0.42