Background: With this case report we want to demonstrate the results of chemotherapy application to the mastectomized side in a patient who had undergone radical mastectomy. Case Report: A patient who was accidentally given chemotherapy on the mastectomized side (PCMS) and a control patient who received chemotherapy on the non-affected side (PCNS) were included in this study. Edema, pain, muscle strength, and shoulder mobility were evaluated. The results of the 2 patients were compared. After chemotherapy, PCMS experienced edema and pain in the affected arm compared to PCNS. Increased circumference measurement, and decreased shoulder mobility and muscle strength were observed in PCMS. Conclusion: It was suggested that chemotherapy application on the mastectomized side triggered lymphedema. Our findings on the subject revealed that education of health care professionals and patients alike is very important.