Conventional methods of assessing adequate nutrition in children may be inaccurate or change too slowly to be helpful during acute illness. Techniques to measure protein synthesis or breakdown provide more accurate information. Three-methylhistidine (3MH), an unusual amino acid found in actin and myosin, is not further metabolized and is quantitatively excreted following muscle degradation. Urinary excretion of 3MH has, therefore, been proposed as a marker of protein catabolism. The ratio of 3MH to creatinine in 24-hr urine collectons was studied in 14 healthy or stressed premature infants, and compared to nitrogen balance (N bal), caloric intake and clinical course. There is a significant inverse correlation between 3MH/Cr ratio and N Bal (R = -.507). The mean 3MH/Cr ratio was 0.140 +/- 0.037 mu mole/mg (n = 37) in healthy growing premature infants and 0.296 +/- 0.160 (n = 26) in infants who were stressed and/or had inadequate nutrient intake. Healthy growing infants almost invariably had a ratio below 0.200. Serial determinations in three infants consistently showed a rise in the ratio to above 0.200 during periods of stress or decreased intake. The 3MH/Cr ratio may be a more sensitive indicator of metabolic status and may be useful clinically, especially in infants receiving total parenteral nutrition.