This study prospectively correlated the level of expression of CD55 on tumours with 7-year survival in 136 colorectal cancer patients. Patients with tumours expressing high levels of CD55 had a significantly worse survival (24%) than patients with low CD55 levels (50%, p<0.02). A similar difference was seen for patients (Duke's B or C) with a high risk of recurrence (29% vs 58%, p<0.05). Furthermore, there was a progressive deterioration in prognosis with increasing antigen expression ( p=0.01). It remains unclear if CD55 is overexpressed by tumours to protect them from complement or if it is related to the recent observation that CD55 is a ligand for the T-cell activation antigen CD97. However, it is a marker of aggression, as colorectal cancer patients whose tumours overexpress CD55 have a significantly reduced 7-year survival.