An excessive requirement for methionine (MET) for growth, termed MET dependence, appears to be a general metabolic defect in cancer. We have previously shown that cancer-cell growth can be selectively arrested by MET restriction such as with recombinant methioninase (rMETase). In the present study, we utilized patient-derived orthotopic xenograft (PDOX) nude mouse models with pancreatic cancer or melanoma to determine the relationship between intra-tumor MET level and tumor size. After the tumors grew to 100 mm³, the PDOX nude mice were divided into two groups: untreated control and treated with rMETase (100 units, i.p., 14 consecutive days). On day 14 from initiation of treatment, intra-tumor MET levels were measured and found to highly correlate with tumor volume, both in the pancreatic cancer PDOX (p<0.0001, R²=0.89016) and melanoma PDOX (p<0.0001, R²=0.88114). Tumors with low concentration of MET were smaller. The present results demonstrates that patient tumors are highly dependent on MET for growth and that rMETase effectively lowers tumor MET.