Food intake regulation is influenced by serum amino acid (AA) ratios. The objectives of this study were to determine whether non-obese subjects with diabetes have normal serum AA ratios, and to see if AA ratios relate to perceived hunger. We therefore measured fasting serum AA in nine normal subjects on two occasions and 19 patients with diabetes on four occasions over a two month period. At the time of the blood samples, the subjects with diabetes were asked to rate how hungry they felt on a scale of -3 (very hungry) to +3 (very full). There were no significant differences between the mean AA ratios of normal and diabetic subjects. There was a positive correlation between reported hunger score and the ratio of serum tryptophan (TRP) to the sum of the concentrations of the other large neutral AA (LNAA) (r = 0.286, n = 78, p less than 0.05). The relationships between hunger and other AA ratios were not significant. Variations of the TRP/LNAA ratio within each subject did not relate to differences in hunger from day-to-day. However, there was a positive correlation between the mean TRP/LNAA ratio and mean hunger score of the different subjects (r = 0.495, n = 19, p less than 0.05). The relationships between TRP/LNAA ratio and body weight or hunger score were not significant. It is concluded that the serum AA ratios are normal in patients with diabetes. The data is consistent with the hypothesis that a low serum TRP/LNAA ratio causes increased feelings of hunger in different individuals.