Malnutrition is a major complication in cancer patients and can be the first symptom to reveal the presence of the disease. The present study aimed to determine the extent of malnutrition among female cancer patients treated by chemotherapy in Makkah and Jeddah, Saudi Arabia. A purposive sample of 72 female cancer patients, aged 39 -52 years, on chemotherapy over 3 month period were subjected to the study. The patients were selected from King Faisal Hospital in Jeddah and King Abdullah Medical City in Makkah Almokarama. Data were collected through an interview with patients using questionnaire for collecting anthropometric measurements, dietary assessment, and some side effects after chemotherapy treatment. Biochemical analysis was also conducted. The results showed that all cancer patients suffered from side effects such as nausea and vomiting after chemotherapy treatment. The percentage of total malnourished patients was higher than nourished patients and recorded 76.19 %, 88.24 % , 83.33 %, and 77.27 % in the breast and ovarian cancers, colon and rectum cancer, lung cancer, and hodgkin cancers and hodgkin's lymphoma, respectively. Malnutrition varied between weight loss, low hemoglobin, low serum protein, decreased serum minerals and impaired liver functions. All patients consumed calories and protein lower than patient requirements. Therefore, the present study recommended that all cancer patients need a good assessment and monitoring to prevent malnutrition and improve health care by using intervention techniques according to patients' status and laboratory results.