A group of forty patients with generalized peritonitis was studied comparing their clinical evolution and mortality with the plasma coloidosmotic pressure values (COP). Twentynine patients survived without complications (Group I) this patients had the higher mean COP values, 17.56 mm Hg at the beggining and 19.06 mm Hg at the end of the study. Three patients showed complications directly related with peritonitis but finally survived (Group II). Their mean COP values were 16.08 mm Hg at the beggining and 13.44 mm Hg at the end. Eight patients died (Group III) lower mean COP values were obtained in this group; 13.5 at the begging and 11.94 at the end. Despite the same kind of medical and surgical treatment, response was very different in each group. Lower values of COP were found as clinical avolution worsened. A very low inicial value or failure to raise it despite the treatment may be considered as a bad prognosis sing.