Seromucoid fraction was measured in terms of mucoid proteins (MP) and hexose content from sera of 1) 47 healthy women, 2) 48 women who had benign breast diseases, 3) 151 untreated patients with breast cancer, and 4) 245 follow-up samples collected from the same breast cancer patients. Mucoid proteins and hexose levels were found to be significantly elevated in untreated patients with breast cancer when compared with the healthy participants (p < 0.001) and patients who had benign breast diseases (p < 0.02 and p < 0.05, respectively). Receiver operating characteristic (ROC) curve analysis revealed potential diagnostic application of both markers for breast cancer. A good correlation was observed between favorable treatment response and decline in serum-marker levels. The markers in patients who did not respond to anticancer therapy remained stable or increased during follow-up. These data indicate that seromucoid fraction can be an useful biochemical marker for breast cancer patients.