The effects of the ketogenic diet (KD) on carnitine levels were determined in 38 consecutive patients with epilepsy treated at Rush-Presbyterian-St Luke’s Medical Center, Chicago, IL Carnitine levels were determined at 0, 1, 6, 12, and 24 months of diet treatment. Reduced total plasma carnitine (TC) at diet initiation was related to multiple antiepileptic drugs (AED), but only 3 had TC deficiency (range, 11.9-21.4 mcM). Treatment with valproic acid was not responsible for the low TC. TC was unrelated to a low blood glucose or time to achieve ketosis. A decrease in TC in the first months of diet treatment was followed by stabilization and a slight increase with long term treatment. None developed clinical signs of carnitine deficiency, and only 19% of patients received supplements of carnitine. An increased acyl/free carnitine ratio is dependent on the level of ketosis during KD treatment, and is not a useful measure for carnitine insufficiency. 
COMMENT. The majority of patients on the ketogenic diet do not develop low carnitine plasma levels, and those with low levels are asymptomatic. TC should be checked at 1 to 6 months of treatment, especially for patients with low-normal TC at KD initiation.