Purpose: We assessed the absorbed dose to the tumor (Dosetumor) by using pretreatment FDG-PET and whole-body (WB) planar images in repeated radioimmunotherapy (RIT) with 131I rituximab for NHL. Materials and Methods: Patients with NHL (n=4) were administered a therapeutic dose of 131I rituximab. Serial WB planar images after RIT were acquired and overlaid to the coronal maximum intensity projection (MIP) PET image before RIT. On registered MIP PET and WB planar images, 2D-ROIs were drawn on the region of tumor (n=7) and left medial thigh as background, and Dosetumor was calculated. The correlation between Dosetumor and the CT-based tumor volume change after RIT was analyzed. The differences of Dosetumor and the tumor volume change according to the number of RIT were also assessed. Results: The values of absorbed dose were 397.7±646.2cGy (53.0～2853.0cGy). The values of CT-based tumor volume were 11.3±9.1 cc (2.9～34.2cc), and the % changes of tumor volume before and after RIT were -29.8±44.3% (-100.0%～+42.5%), respectively. Dosetumor and the tumor volume change did not show the linear relationship (p>0.05). Dosetumor and the tumor volume change did not correlate with the number of repeated administration (p>0.05). Conclusion: We could determine the position and contour of viable tumor by MIP PET image. And, registration of PET and gamma camera images was possible to estimate the quantitative values of absorbed dose to tumor. (Nucl Med Mol Imaging 2009;43(1):60-71)