Several studies reported that cancer is less frequent in persons with Alzheimer's and Parkinson's Diseases (AD/PD) and vice-versa. We evaluated whether a different distribution of known nongenetic risk factors for cancer and AD/PD, might explain their inverse relationship of occurrence. We nested 2 case-control studies in a subsample of a large cohort of 1,000,000 resident in Lombardy Region in Italy (n=1515), followed-up for cancer and AD/PD occurrence since 1991 until 2012. Conditional logistic regression was performed to determine the odds ratios (OR) and 95% confidence intervals (CI) of AD/PD in subjects with and without cancer and the risk of cancer in those with and without AD/PD. A total of 54 incident cases of AD/PD and 347 cancer cases were matched with 216 and 667 controls, respectively. After controlling for low education, obesity, history of hypertension, diabetes, dyslipidemia, physical activity, smoking habit, alcohol consumption, and dietary habit, cancer was found inversely associated with the risk of AD/PD (OR, 0.66; 95% CI, 0.32-1.38), and the risk of cancer in AD/PD was similarly reduced (OR, 0.42; 95% CI, 0.20-0.91). Different exposures to nongenetic risk factors of both diseases do not explain their competitive relationship of occurrence.