Background: Low levels of public knowledge, incorrect beliefs, and anxiety are the most often mentioned factors that may negatively affect the implementation of preventive campaigns and timely diagnosis of cancer. Cancer is a major unresolved problem for global public health. As a result, many effective preventive measures need to be found and implemented. Methods: For a duration of 18 months, readers of the Polish scientific Internet portal were invited to participate in the Polish On-line Randomized Intervention aimed at Neoplasm Avoidance (PORINA) study. Level of cancer-related anxiety was our main measure (self-declared on a simple five-point Likert scale) in this analysis. Results: A total of 463 participants were qualified for the final analysis. Respondents with a positive family history of cancer ( p < 0.001) declared the highest level of cancer-related anxiety, whereas lower levels were declared by those previously treated for cancer ( p = 0.006). The conducted educational intervention reduced the declared level of cancer-related anxiety. Conclusions: The results of this study provide evidence that the use of web-based interventions aimed at increasing awareness could reduce cancer-related anxiety and may lead to more frequent consent to undergo some of the medical procedures used to diagnose or treat cancer.