NCI investigators have recruited approximately 50,000 adults in Golestan Province who are now under active follow-up. The focus of the cohort is to evaluate environmental and genetic risk factors for esophageal squamous cell carcinoma, but other diseases and conditions will also be investigated. All subjects are actively contacted every 12 months, and investigators plan to follow enrolled subjects for at least 10 years. Investigators will also evaluate other leading causes of death such as heart disease and stroke. The Golestan Cohort Study is the first large, population-based prospective study in Western or Central Asia that has detailed exposure assessment, biological samples, and virtually complete follow-up data. Over the coming decade, it should provide important information about the etiology of esophageal cancer, gastric cancer, other cancers, cardiac diseases, and stroke in this population. In a recent publication, investigators demonstrated that opium users have an increased risk of death from multiple causes compared with non-users.