MRI-guided cryoablation of small renal tumors relies on achieving sufficient ice ball coverage throughout the entire lesion for effective treatment. However, a volumetric approach to assess treatment margins intraoperatively is currently lacking. This work retrospectively derived three-dimensional ablation margins after cryoablation in 39 kidney tumors using co-registration of intraoperatively acquired pre- and post-ablation MR images. Minimal ablation margins were significantly smaller for cases with versus without local tumor progression. Radiological complete coverage (ablation margin >0 mm) was associated with local control. MRI-derived 3D ablation margins may present a useful intraprocedural tool to assess treatment success.