Simple SummarySignet ring cell (SRC) carcinoma of colorectal and appendiceal cancer is rare but is recognized as the histopathological subtype with the poorest prognosis. However, the prognostic relevance of a SRC component <50% is unclear. The aim of this study was to provide a clinicopathological characterization of all SRC-containing colorectal and appendiceal cancers, including those with <50% SRCs. The results showed that SRCs, both ≥50% and <50%, were associated with aggressive histopathological features, advanced stages, and, particularly, peritoneal metastases. Information about the presence of SRCs in tumour tissue, not only in the case of ≥50% SRCs, should be routinely registered in pathology reports and clinical registers to enable larger studies that can aid our understanding of SRCs in colorectal and appendiceal cancers.AbstractBackground: Colorectal signet ring cell (SRC) carcinoma with ≥50% SRCs (SRC ≥ 50) has a poor prognosis, but the prognostic role of SRCs < 50% (SRC < 50) is unclear. The aim of this study was to provide a clinicopathological characterization of SRC colorectal and appendiceal tumours and analyse the importance of the SRC component size. Methods: All patients in the Swedish Colorectal Cancer Registry diagnosed with colorectal or appendiceal cancer in 2009–2020 at Uppsala University Hospital, Sweden, were included. The SRCs were verified, and the components estimated by a gastrointestinal pathologist. Results: Of the 2229 colorectal cancers, 51 (2.3%) had SRCs, with a median component size of 30% (interquartile range of 12.5–40) and 10 (0.45%) had SRC ≥ 50. The SRC tumours were primarily localized in the right colon (59%) and appendix (16%). No patients with SRCs had stage I disease, and 26 (51%) had stage IV, of whom, 18 (69%) had peritoneal metastases. The SRC tumours were often high grade with perineural and vascular invasion. The 5-year overall survival (OS) rate for patients with SRC ≥ 50 were 20% (95% confidence interval (CI) 6–70), for SRC < 50, 39% (95% CI 24–61); and for non-SRCs, 55% (95% CI 55–60). Among the patients with SRC < 50 and <50% extracellular mucin, the 5-year OS was 34% (95% CI 19–61), while those with ≥50% extracellular mucin had an OS of 50% (95% CI 25–99). The 5-year recurrence-free survival rates were 51% (95% CI 13–83) for patients with SRC tumours, as compared to 83% (95% CI 77–89) and 81% (95% CI 79–84) for mucinous and non-mucinous adenocarcinoma, respectively. Conclusions: The presence of SRCs was strongly associated with aggressive clinicopathological features, peritoneal metastases, and poor prognosis, also when they make up <50% of a tumour.