To analyse the causes of non resectability of peritoneal carcinomatosis (PC) of non-appendiceal colorectal carcinomas, discovered only at the time of the laparotomy. The combination of a maximal cytoreductive surgery (resecting tumor deposits > 1 mm in diameter) with intraperitoneal chemohyperthermia results in cure a significant number of patients. Complete resection of the PC is the determining factor of this time-consuming and resource-consuming therapy. Unfortunately, it has not been possible, so far, to safely predict complete resectability before carrying out the laparotomy. All patients with colorectal PC who had undergone a laparotomy in order to receive this new treatment, but who finally presented a non completely resectable PC were included in our study. Their preoperative parameters were retrospectively studied and compared to matched number of patients who had successfully undergone this treatment. 29 patients had incomplete resection PC at laparotomy. They were compared with 29 matched patients who underwent a complete resection of the PC. The factors predicting non resectability were, in decreasing order of frequency: presence or persistence of an ascitis just before the laparotomy (P = 0.0008), progression of the PC while on neo-adjuvant chemotherapy (P = 0.01), abnormal CT- imaging (P = 0.03), and sub-occlusive syndrome (P = 0.05). These parameters were partially inter-related. The persistence of ascitis and any progression of the disease while on chemotherapy are important predictive factors of incomplete resectability of non-appendiceal colorectal PC.