Background: Diffusion-weighted magnetic resonance imaging (MRI) is an advanced functional imaging method based on the molecular motion of water. The diffusion of the water molecules in the tissue is dependent on a number of factors. One factor that modulates the diffusion of water molecules is the cell density in the tissue and the nuclear/cytoplasmic volume ratio, which is often increased in high-grade tumors. Aim: In line with this information, the present study aimed to investigate the role of apparent diffusion coefficient (ADC) values in the differential diagnosis of brain tumors and to demonstrate their contribution to diagnosis. Materials and Methods: We measured ADC values in three different regions of the lesion in 317 patients with central nervous system tumors on diffusion-weighted imaging (DWI) and compared the mean calculated ADC values. Results: In our study, ADC values of low-grade glial tumors were significantly higher than all other groups. In the comparison between metastatic patients, the ADC values of the patients with lung metastasis were considerably higher than the ADC values of the patients with metastasis other than breast and lung metastases. In addition, in comparing lung subtypes, we found that the ADC values of patients with histopathological diagnosis of small cell lung cancer (SCLC) were significantly lower than other subtypes. There was no significant difference between high-grade glial tumors and metastases. The number of patients in some subgroups in the metastasis group was insufficient. Therefore, we could not perform statistical analysis, but the mean ADC values of patients with renal cell carcinoma (RCC) metastasis were approximately 0.521 × 10 −3 mm 2 /s and were lower than the ADC values of other metastatic tumors. Conclusion: In the differential diagnosis of central nervous system (CNS) tumors, DWI makes important contributions to conventional MRI examinations. If supported by studies with larger and homogeneous groups, more precise and reliable results can be obtained with the help of other imaging findings.