Six glycogen-rich clear-cell carcinomas (GCC) were found among 439 cases of breast cancer (BC) in a thorough search among a defined urban population. Five of these six patients had axillary lymph node metastases at diagnosis and all five died from their breast cancer within 7 years following the diagnosis. Tumors with histologic features of GCC were larger (P = 0.03), and they had a large DNA index (greater than 1.3) in flow cytometric DNA analysis more frequently than BCs in general (P = 0.04). All GCCs were nondiploid and had a high S-phase fraction (greater than 9%, mean 19.2%), which suggests that BCs with glycogen-rich cell features may be more aggressive than BCs in general.