The Da-sein of Colorectal Cancer is an empirical-phenomenological study, which aimed at describing the experience of having received a diagnosis of colorectal cancer and what this means for the person concerned. Heidegger's Ontology of Dasein (1993; 1996) was selected as the philosophical frame of reference. In this study, dialogues, field notes and a diary were used as methods for data collection. Data analysis followed Colaizzi's analytic method as described by Haase (1987). Over all, seven patients who had been diagnosed with colorectal cancer were followed over a time-span of 13 months. The findings exposed that uncertainty, transitoriness, as well as locus of control are highly important in individual illness management. Upon receiving a diagnosis of colorectal cancer and subsequent therapeutic treatments uncertainty and transitoriness already become determining elements. Despite successful completion of the anti-cancer therapies, neither uncertainty, nor transitoriness will disappear completely from the patients' life. Upon disclosure of a diagnosis of colorectal cancer, the person concern defers to an external locus of control. Patients accept decisions taken by others or delegate powers of decision making. After completion of the anti-cancer therapies, returning to everyday life becomes the top priority. To find their own rhythm of life again patients work at regaining an internal locus of control. Powers of decision making are retrieved. Through The Da-sein of Colorectal Cancer the period directly before and after the disclosure of the diagnosis was investigated. The newly acquired knowledge will contribute to a better understanding and facilitate care of this patient group.