AbstractNeurofibromatosis Type 1 (NF1) is a frequent tumor suppressor gene disorder characterized by multiple benign tumors and high risk of malignancy. Internal tumor burden is a major disease‐associated manifestation and can be most adequately assessed by magnetic resonance imaging of the whole body. Approximately 5% of NF1 patients have constitutional large NF1‐deletions that are generally associated with more severe clinical manifestations. Here, we investigated whether these deletion patients also have more and/or larger internal tumors by assessing internal tumors and their total volume (exclusive of cutaneous and subcutaneous) in 38 NF1 deletion patients (including eight mosaic cases) and 114 age‐ and gender‐matched NF1 patients without deletions. The incidence of internal tumors was significantly lower in mosaic deletion patients (1/8 = 13%) but did not differ between the 30 nonmosaic deletion patients and the 90 age‐ and gender‐matched NF1 patients without large deletions used as controls. Neither the number nor the total volume of tumors per patient differed significantly between the latter two groups. However, extremely high tumor burden (>3,000 ml) was significantly more frequent among nonmosaic NF1 deletion patients than among NF1 patients without large deletions (13% vs. 1%, P = 0.014). Thus, as a group, patients with NF1 deletions do not exhibit a significantly higher internal tumor burden than NF1 patients without such deletions. However, deletion patients can frequently have extremely large internal tumors and thus demand special attention. © 2012 Wiley Periodicals, Inc.