To analyze the biology of small (20 mm or less) advanced colorectal carcinomas (SAC), 24 cases, 22 small early colorectal carcinomas (SEC) of similar size, and 52 advanced colorectal carcinomas (AC) were studied. The proliferative (Ki-67) labeling index for SAC was 65.9+/-17.1%, significantly higher than those for SEC (30.9+/-13.7%) or AC (43.0+/-17.1%) (P < 0.01). Matrix metalloproteinase (MMP)-9 and tissue inhibitors of metalloproteinase (TIMP)-2 expressions for SAC were 62.5 and 79.2%, respectively, significantly higher than those for SEC (4.5, 13.6%) or AC (21, 33%) (P < 0.01). Small advanced carcinomas have higher invasiveness than SEC or AC and may represent a different type of cancer.