The use of methylsulfonylmethane (MSM), available as an "over-the-counter" dietary supplement, led to the occurrence of an abnormal resonance at 3.15 ppm in the in vivo brain proton MR spectrum as well as the in vitro cerebrospinal fluid NMR study of a 4-year-old girl. The concentration of this compound amounted to 1.2 mmol/l in brain tissue and 1.7 mmol/l in cerebrospinal fluid. Our findings illustrate that ingestion of exogenous compounds, e.g., in medication, food or "innocent" supplements, may lead to abnormal resonances in spectroscopy studies that might be difficult to assign.