This article reports on the first double-blind randomized clinical study with an antiidiotype antibody vaccine in patients with metastatic colorectal carcinoma. The study was performed to determine immunological parameters, efficacy, and tolerability of the vaccine. Forty-two patients with metastatic colorectal cancer were randomly assigned to multiple immunizations with goat IgG antiidiotype vaccine SCV 106 (n = 21) or unspecific goat IgG as controls (n = 21). The antiidiotype vaccine mimicked the 17-1A glycoprotein antigen associated with colorectal cancer. Of the 42 patients entered, 39 were evaluable for efficacy (SCV 106, n = 18; controls, n = 21). Twenty-nine patients raised antibodies to the vaccines (immunological responders, SCV 106, n = 12; controls, n = 17). Only in the SCV 106 group was a significant increase (p = 0.002) of titers with specificity of antitumor antibody 17-1A found. According to the International Union Against Cancer (UICC) criteria no tumor response was observed. However, in the SCV 106 group the relative increase of carcinoembryonic antigen (CEA) levels between entry and observed disease progression was lower (p = 0.03) and disease progression was determined less frequently by development of new metastases (p = 0.001). On an intention-to-treat basis, the survival time difference between the two groups was not significant. Comparison of immunological responders in both groups revealed a significant survival advantage of the SCV 106-treated patients compared with controls (mean 67 versus 39 weeks; p = 0.01). Immunizations were well tolerated. Vaccination of immunologically responding metastatic colorectal carcinoma patients with SCV 106 leads to slowed disease progression and tumor dissemination and significantly prolongs survival time.