Glioblastoma multiforme (GBM) is an aggressive primary brain malignancy with a very poor prognosis. Researchers employ animal models to develop potential therapies. It is important that these models have clinical relevance. This means that old models, propagated for decades in cultures, should be questioned. Parameters to be evaluated include whether animals are immune competent or not, the infiltrative growth pattern of the tumor, tumor volume resulting in symptoms and growth rate. We here describe the growth pattern of an experimental glioblastoma model in detail with GFP positive glioblastoma cells in fully immune competent animals and study tumor growth rate and tumor mass as a function of time from inoculation. We were able to correlate findings made with classical immunohistochemistry and MR findings. The tumor growth rate was fitted by a Gompertz function. The model predicted the time until onset of symptoms for 5000 inoculated cells to 18.7±0.4 days, and the tumor mass at days 10 and 14, which are commonly used as the start of treatment in therapeutic studies, were 5.97±0.62 mg and 29.1±3.0 mg, respectively. We want to raise the question regarding the clinical relevance of the outline of glioblastoma experiments, where treatment is often initiated at a very early stage. The approach presented here could potentially be modified to gain information also from other tumor models.