This study was carried out to determine the changes in sexual functioning of patients with cancer. We identified the frequency and causes of changes in sexual functioning among patients with cancer, and determined whether this change differed according to age, type of cancer, type of treatment, side effects of drugs, anxiety, depression, and perceived social support. The study was conducted with 192 patients with cancer. Data were collected using the data collection form which included questions about socio-demographic, disease-related characteristics as well as those related to changes in sexual functioning. In addition, the Hospital Anxiety and Depression Scale and the Multidimensional Scale of Perceived Social Support Scale were used. Data were analyzed using the SPSS 15.0 software. For analyzing the data, means, frequencies, percentages, t test and Pearson's Chi square test were used. Of the patients with cancer, 69.3% were experiencing changes in sexual functioning, where 75.9% of those began to have changes in sexual functioning only after the cancer. Experiencing changes in sexual functioning after cancer did not differ according to the type of cancer (when genitourinary cancers and breast cancer were compared to the other cancer types) (p > 0.05). Experiencing changes in sexual functioning after cancer significantly differed according to age, the presence of fatigue, and depression scores (p < 0.05). The mean age of those who experienced changes in sexual functioning after the cancer was lower than those who did not, their fatigue rate and depression scores were higher. It is suggested that healthcare professionals should view sexual functions as vital functions such as movement, eating, and respiration and evaluate patients with regard to those functions.