Cancer is a disease that affects many people. Therefore, issue of proper psychological adaptation to the disease is of utmost importance and allows one to look for a better understanding in similar fields. Cancer has many common features with chronic diseases, despite the fact that it is not established whether it can be classified as a chronic di­sease. The first goal of this article is to confirm that according to the WHO definition, cancer indeed can be classified as a chronic disease. The second aim is to present implications of this assumption. Knowledge about the functioning of patients with chronic disease may be a source of hypotheses for psychological adjustment in cancer. Considering cancer as a chronic disease allows application of psychological understanding of chronic diseases to the study of cancer.