Ketogenic diet (KD) is highly effective in controlling epileptic seizures in children. One of the mechanisms postulated, the accumulation of ketone bodies, acetoacetate (AcAc) and/or betahydroxybutyrate (beta-OHB) in the brain, would be detectable by non-invasive proton magnetic resonance spectroscopy (1H-MRS). 1H-MRS was performed in occipital cortical grey matter in 14 epileptics (E); ages 3.8-48.3 years), seven KD and seven without, and 16 healthy age-matched subjects. One E was examined before and after KD. A singlet resonance (sigma = 2.22 ppm), distinct from AcAc (sigma = 2.26 and 3.46 ppm), and identified as acetone was present in all spectra of children on KD (nine spectra in five children; concentration 0.7 +/- 0.2 mM). This resonance was absent from Control and E without diet. AcAc and beta-OHB, which were not detectable in KD brain, were found in urine or blood of all KD. Seizures were well controlled in all E in whom acetone was detected. Two of seven E, both adults, were seizure-free without detectable acetone. Cerebral acetone may contribute to seizure control in KD, but is unlikely to be the sole mechanism.