A sandwich ELISA system for detecting vascular basement membrane associated collagen (BAC) was developed. Serum levels of BAC were determined in patients with liver diseases (N = 53), various cancers (N = 65) and other diseases (399). Serum levels of procollagen type III (PIIIP) amino propeptide, type IV collagen.7s domain (7s domain) and other parameters (TP, ALB, GOT, GPT, CHE, gamma-GTP, ALP, LDH, CHE, TG, GLU) were also determined in those patients. In the whole patients, serum concentrations of BAC showed a weak correlation with GOT, GPT, ALB and CHE but not with gamma-GTP and ALP. There was no correlation between BAC and PIIIP or 7s domain. Although serum levels of BAC were elevated in both liver diseases and cancers, the increase in liver diseases was more marked. Markedly increased serum levels of BAC with low levels of CHE were found only in liver cirrhosis and liver cirrhosis plus hepatocellular carcinoma. Increased BAC may reflect capillarization of the liver sinusoid or remodeling of the vascular basement membrane which is observed in the progression of liver fibrosis. Serum BAC is thought to be a promising new marker, different from PIIIP or 7s domain for diagnosing fibrosis state in the organs, particularly in the liver.