Urinary cyclic adenosine monophosphate levels were measured in 150 patients with accidental injury of varying causes. Thirty-eight healthy adults of both sexes serving as controls excreted 2.21 to 6.85 micromoles of cyclic adenosine monomphosphate per gram of creatinine, mean 4.34 +/- 1.25. In 120 patients with trauma on admission, the excretion was increased by 15.7 per cent, p less than 0.05, and the changes showed a time related pattern. In patients admitted within the first 30, 60 and 120 minutes after trauma, the mean excretion was changed by 19, 10 and minus 2.8 per cent, respectively, and in those admitted between two and 24 hours by 30 per cent. Twelve patients with differing types of trauma showed a mean 24 hour excretion reaching its peak on the first day, 44 per cent, and declining to its nadir of 2.25 micromoles per gram of creatinine on the third day, minus 47 per cent, p less than 0.01. A second rise reached its peak on the fifth day, p less than 0.05. Thereafter, the excretion fluctuated widely with peaks significantly above and below the control range, and we were unable to correlate these changes with any specific factors. In post-traumatic acute renal failure, the nucleotide excretion fell within several hours and usually reached low values, that is, below 0.25 micromoles per 1,000 milliliters per 24 hours within one to three days. In general, the excretory pattern for cyclic adenosine monophosphate followed that of creatinine clearance, but in the diuretic phase of the recovering kidney, the cyclic adenosine monophosphate levels remained more depressed than those of creatinine. The high sensitivity of urinary cyclic adenosine monophosphate to abnormalities in renal function suggests its potential as a clinical indicator.