Carnitine is necessary for the translocation of fatty acids into the mitochondria, and the relative concentration of carnitine and acylcarnitine in the serum are known to reflect metabolic states. A survey of serum carnitine concentrations was made in 54 cancer and 81 noncancer patients for the purpose of determining the carnitine profile. The total carnitine, nonesterified carnitine, and acid-insoluble acylcarnitine concentrations of cancer patients were similar to noncancer patients and within the normal range; however, the acid-soluble acylcarnitine concentration was significantly lower in cancer patients than in controls (6.7 vs 11.5 nmol/ml). When percentages and ratios were calculated for the relative proportions of acylcarnitines, large variations were found to occur among cancer types. The acylcarnitine ratio (the sum of acid-soluble and acid-insoluble acylcarnitine divided by nonesterified carnitine) ranged from 0.17 in leukemia to 0.30 in breast cancer cases. Since the acylcarnitine concentration and ratio are reflective of the metabolic state, the depressed acylcarnitine ratio in cancer patients may be due to decreased production, increased utilization, or increased excretion of acid-soluble acylcarnitine. Elevated concentrations of nonesterified carnitine and total carnitine were observed in two patients, and some of the lowest acylcarnitine concentrations and ratios were observed in advanced cancer cases. The therapeutic regimen and/or the neoplastic process itself may be responsible for the observed differences in the serum carnitine profile.