Strangulation obstruction is a surgical emergency, but its accurate diagnosis and timely surgical treatment are still matters of debate. We conducted a prospective observational study. We performed diagnostic paracentesis preoperatively for patients with suspected strangulation obstruction or, if that was impossible, we obtained ascites at the time of laparotomy. We examined each specimen to see whether ascites color and laboratory parameters could be reliable indicators of strangulation obstruction. During 18 months, 32 patients had suspected strangulation obstruction. At laparotomy, we confirmed strangulation obstruction in 21 patients, simple obstruction in two patients, and pseudo-obstruction in one patient. We treated eight patients conservatively, including one patient with a complication. We identified ascites red blood cell count, hematocrit, and lactic acid as indicators for strangulation obstruction by univariate analysis. An ascites red blood cell count was statistically high in cases with strangulation obstruction by multivariate analysis. Ascites red blood cell count above 20,000/mm(3) had a positive predictive value for strangulation obstruction of 100%, and above 40,000/mm(3), bowel resection was highly necessary. Diagnostic paracentesis and ascites analysis are useful methods for diagnosis of strangulation obstruction. Diagnostic paracentesis and ascites analysis should be combined with careful clinical exams for diagnosis of strangulation obstruction.