Obesity is associated with a risk of at least 20 different cancers. We aimed at defining cancer risks in prospectively recruited patients with a novel subgroup, those with a family history of obesity. We defined a cohort of 30 020 patients who had been hospitalized since 1964. Cancer risks in these patients were followed through 2006. Standardized incidence ratios were calculated for cancer using those not hospitalized for obesity as a reference population. We could also identify persons who had been hospitalized for type 2 diabetes. A total of 1721 patients were diagnosed with cancer after hospitalization for obesity, showing an increased risk for 12 cancers and a decrease for breast cancer. The largest increases were found for nervous system hemangioma (13.64, 95% confidence interval 2.57-40.37) and other male genital (3.94, 1.24-9.26), bone (3.41, 1.23-7.47), small intestinal (2.93, 1.60-4.93), kidney (2.46, 1.97-3.02), and endometrial (2.32, 2.01-2.66) cancers. Among endocrine cancers, adrenal tumors showed the highest risk, of 3.74 (1.86-6.72). The overall risk was 1.19 (1.13-1.25). Family history of obesity was associated with formerly unrecognized increased risks of gallbladder and colon cancers and ocular melanoma. Cancer risks in this relatively young obese population differed quantitatively from those found after type 2 diabetes. The novel findings included rare and relatively benign tumors, probably found in endocrinological and other medical examinations for obesity and related conditions. Similarly, male genital cancer may be related to sexual behavior, and bone cancers, found in old individuals, could be related to propensity for fractures.