Gastrointestinal cancers describe malignant changes that affect the organs of the digestive system and its ancillary organs and are responsible for more cancers and cancer-related deaths than any other system in the body. Nickel is widely used in various industries, causing many human cancers through different mechanisms. Our present study is designed to estimate nickel concentrations in serum samples from patients with different types of gastrointestinal cancers. The study was carried out in 45 patients with many types of gastrointestinal cancers at Al-Biruni University Hospital for Cancerous Diseases in Syria compared with 35 healthy controls. All measurements of serum were done with a graphite atomic absorption spectrophotometer equipped with a deuterium lamp. Our results showed that patients with gastrointestinal cancers had significantly higher serum levels of Ni than healthy controls (p < 0.0001), and there were no statistically significant differences between the four types of gastrointestinal cancers included in the study (p > 0.05). The increase in serum nickel concentrations has a direct effect on the incidence of gastrointestinal cancers by influencing the mechanisms that induce cancer by nickel.