Despite the rapid increase in the number of cancer patients in Kenya, the immense physical and psychological challenges that this diagnosis brings both during and after treatment remain largely unaddressed. Literature shows that cancer illness is greatly feared. The fear may influence self disclosure by patients regarding the effects of cancer illness to people they interact with in their day-to-day living. Self-disclosure involves expressing personal experiences on emotions, thoughts, and wishes as a result of cancer diagnosis, verbally to significant others. Although such self-disclosure has been shown to reap psychological benefits, the action can be hampered by stigma and fear around cancer. Self-disclosure is also required in order to enlist and secure support during and after treatment of cancer. The main objective of this study was to determine the influence of fear of cancer illness on self-disclosure among adult patients at Kenyatta National Hospital Cancer Treatment Center (KNH-CTC) in Nairobi City County, Kenya. The study whose target population was 3500 adult cancer patients who annually got treatment and review at KNH-CTC, was informed by the Social Cognitive Processing Model of Emotional Adjustment to Cancer and employed a descriptive survey research design. A sample size (n=347) was determined using the Cochran formula. Non-probability sampling, using purposive sampling method was used to select the study participants. Questionnaire and Focus Group Discussion were used to collect data. Both validity and reliability of the research instrument were established before conducting the study. In addition, all research ethical and authorization protocols were observed. Descriptive and inferential statistics were used for quantitative analysis of data while thematic analysis was used to analyze qualitative data. The findings revealed that: generally participants had moderate to high levels of fear (n=199; M=57.4; SD=12.072); there was a statistically significant association between levels of fear and levels of self-disclosure, (χ2 (4) = 11.722, p = .020) and that the highest levels of fear were experienced upon diagnosis but reduced in the course of treatment and over time.