We correlated the in vivo 1 H Magnetic Resonance Spectroscopy (MRS) concentration of the choline peak (CHO) with in vitrochemical measures of choline-containing compounds and a histological grade of cellularity in 18 patients with neoplastic and infectious brain lesions. Gas-chromatography-mass-spectrometry (GCMS) was used to measure the concentrations of free choline (Cho), glycerophosphocholine (GPCho), phosphocholine (PCho) and phosphatidylcholine (PtdCho) from biopsies in the same area where MRS was performed. Cellular density, free Cho, PCho and GPCho were the strongest determinants of 1 H MRS CHO while PtdCho was not. Just as the 1 H MRS 2.0 peak reflects both the concentration of n-acetyl-l-aspartate and neuronal density, the 1 H MRS 3.2 peak reflects the concentration of water-soluble choline-containing compounds and cellular density.