Introduction: Chemotherapy is one of the modalities of systemic therapy. It is not tumor-specific and is linked to the occurrence of toxicities in healthy tissues. Chemotherapy also impairs the immune system, reducing the number of immune cells and weakening its function. Aim: This study evaluated immune system parameters among solid tumor patients that underwent chemotherapy before and after three cycles. Parameters that were evaluated here were neutrophil count, lymphocyte count, and CD4 and CD8 pre- and post-three cycles of chemotherapy. Materials and methods: This was a pre- and post-study that took place in the Hematology Medical Oncology Clinic between June 2023 and May 2024. A total of 36 subjects were recruited for this study. Results: Most subjects were female (58.3%) with a mean age of 46.69±16.30 years. Body mass index (BMI) fell primarily into the normal weight range (38.9%), with underweight and overweight coming in second and third, respectively, at 33.3% and 27.8%. Most subjects had stage IV disease (61.1%) and head and neck cancer (30.6%). We evaluated four immune parameters after three rounds of chemotherapy and found that the neutrophil count (p=0.035), lymphocyte count (0.032), CD4 (0.027), and CD8 (0.020) had decreased. A correlation between neutrophils and ECOG score (p=0.021) and a decreased lymphocyte count with weight loss greater than 3 kg (p=0.038) was found in the analysis of clinical characteristics and immune parameters following chemotherapy. CD8 levels were lower in people over 40 (p=0.027). Conclusions: Immune markers such as neutrophils, lymphocytes, and CD4 cells have been shown to decrease as a result of chemotherapy. Following chemotherapy, patients with solid tumors who are older than 40, have an ECOG score of 1 or higher, and have lost at least 3 kg of weight typically exhibit lower immune parameters. Copyright by authors.