Positron emission tomography (PET) using ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) is a cornerstone in oncological imaging for tumor localization and therapeutic assessment. Despite its diagnostic utility, limited attention has been given to its transient effects on hematological parameters. This study evaluates hematologic and immunological alterations following FDG administration by analyzing changes in red blood cells (RBCs), white blood cells (WBCs), and platelets before and after injection. The experimental cohort included one healthy subject and three cancer patients (liver, breast, and uterine). Peripheral blood samples were obtained pre- and post-FDG injection, with subsequent blood film analysis assessing RBC aggregation, WBC distribution, platelet morphology, and indicators of blood viscosity. Findings revealed reversible hematological changes, notably increased RBC aggregation, suggesting transient hyperviscosity potentially driven by oxidative stress. Differential leukocyte analysis revealed neutrophilia, lymphopenia, and monocytosis, suggesting immune modulation. Additionally, a mild rise in platelet aggregation suggested a temporary prothrombotic state. The study concludes that FDG-PET elicits short-term hematological and immunological shifts. These outcomes highlight the importance of cautious interpretation of PET findings in oncology settings and emphasize the need for further investigations into oxidative and immunomodulatory effects to enhance patient safety and diagnostic accuracy.