Objective: The present research was designed with the aim to detect insomnia in cancer patients and to determine the factors responsible for the development of insomnia through patient information and to provide suggestions for any contribution. Material and Methods: The cross-sectional study was conducted on 264 patients including 132 females and 132 males who attended the Outpatient Therapy Unit of Medical Oncology Department of Medical Faculty Hospital within Ankara University and accepted to take part in the research. The questionnaire prepared by the researchers was implemented through personal interview technique and information was collected from the patient files. The questionnaire form includes demographic information, Pittsburgh Sleep Quality Index (PSQI), and Beck's Depression Inventory. Results: Higher prevalence of insomnia was detected in cancer patients (54.4%). Depression negatively affects the sleep quality and the problem of insomnia in the patients, who were diagnosed with depression before, was remarkable. Furthermore, the female participants were more prone to develop insomnia and depression (61.36%, 25.76%, respectively). The sleep duration was found to be affected positively in patients receiving cisplatin containing regimens (p=0.029) whereas those receiving other platin group drugs were affected negatively (p=0.017). The use of pyrimidine and folic acid group drugs positively affected sleep quality (p=0.031). Conclusion: It can be said that sleeping and mental problems of the patients visiting the oncology clinics should be monitored and cared closely.