Urinary amino acids were measured in 10 severely burned patients weekly during 4 weeks after burning. For control the same measurements have been made in 12 multiple injured patients during the first two weeks after the accident. In burned patients the renal loss of free amino acids was up to 3 g/24 h +/- 1 in the first week, together with a high excreation of sodium (450 mmol/24 h +/- 73) in the first week. Urinary output of ornithin (highest value in the 2nd week: 1093 +/- 525% of normal) and lysin (highest value in the 2nd week: 1093 +/- 525% of normal) and Lysin (highest value in the 1st week: 654 +/- 166% of normal) was increased and caused an augmented loss of cystin (645 +/- 133% of normal in the first week) and 742 +/- 64% of normal in the second week. In the first and second week after burning the excretion of valin was up to 12 times the normal. Methionine-excretion staid high (5 times the normal) during all 4 weeks. The mean rate of 3-methylhistidine output in urine (80 +/- 38 mg/24 h) was elevated from the second until fourth week, suggesting muscle protein breakdown. In the control group urinary excretion of glutamin and asparagin (1st week: 608 +/- 97% of normal), threonin (2nd week: 693 +/- 240% of normal), serin (2nd week: 517 +/- 232% of normal) and histidin (1st week: 358 +/- 115% of normal) was high. Only urinary citrullin, isoleucin, prolin and taurin were not elevated in both groups during all 4 weeks.