Several poly(methyl methacrylate) samples of different molecular weight and dispersity were prepared and studied by positron lifetime spectroscopy. According to the lifetime measurements, the free volume holes were smaller and their distribution was narrower in monodisperse PMMA. samples than in polydisperse samples. On the other hand, density measurements revealed that the overall free volume fractions are identical in both kinds of samples. To check these results, we constructed a model for the polymer structure and calculated free volume distributions in it. Theoretical results have not shown dispersity-dependent free volume distributions which suggested that the structure of compressed PMMA pellets differs from the ideal interaction-free structure considered in calculations. This latter result emphasizes the role of entanglements in the formation of the polymer structure.