Aim of the work: To evaluate the role of diffusion weighted imaging (DWI) in the planning for CT guided biopsy of lung and mediastinal lesions. Subjects and methods: This cross sectional study included 23 patients with mean age was 42.7 years came with clinical and imaging suspicion of chest malignancy, referred for CT guided transthoracic biopsy. DWI and apparent diffusion coefficient (ADC) were evaluated for all patients to choose the area of maximum restriction. Results: The study showed that with DWI guidance; all the biopsy specimens were diagnostic and provide adequate material for analysis. Conclusion: DWI and measurement of ADC value has a very helpful role in the pre-procedural planning of transthoracic biopsies.