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Transforming growth factor-β1 (TGF-β1) acts as a potent inhibitor of cell growth and tumor progression but loss of this negative regulation can contribute to tumor development. Some studies have reported an association between disease progression and TGF-β1 expression in patients with colorectal carcinoma, but their results were not always consistent. Serum levels of TGF-β1 were measured using an enzyme-linked immunoadsorbent assay in 121 consecutive patients with colorectal carcinoma and compared with TGF-β1 serum levels in 31 healthy volunteers. Serum levels of TGF-β1 also were measured in 50 patients who underwent curative surgical resection (part of the 121 preoperative patients) to compare their levels with preoperative serum levels of TGF-β1. Serum levels of TGF-β1 in patients with colorectal carcinoma (45 ± 15 ng/mL) (mean ± the standard deviation) were significantly higher than those in the healthy control group (32 ± 4 ng/mL) (P = 0.001). Serum levels of TGF-β1 increased with increasing tumor stage (P < 0.01). Serum levels of TGF-β1 were correlated significantly with depth of tumor invasion, lymph node metastasis, distant metastasis, and serum levels of carcinoembryonic antigen (CEA). Serum levels of TGF-β1 tended to increase with increasing CEA (correlation coefficient = 0.21; P < 0.05). The mean serum level of TGF-β1 in patients with colorectal carcinoma before surgery (45 ± 14 ng/mL) (n = 50) significantly decreased to 34 ± 7 ng/mL, which was within the normal range (32 ± 4 ng/mL), after curative surgical resection of the tumor (P = 0.0000). Serum levels of TGF-β1 after tumor resection decreased more significantly in patients with higher preoperative levels of TGF-β1 (from 53 ± 12 ng/mL to 36 ± 6 ng/mL) (n = 30). The results of the current study suggest that serum levels of TGF-β1 in colorectal carcinoma patients may be associated with disease progression and may be used as a biomarker in the management of colorectal carcinoma patients. The authors believe further studies with a large number of patients for a longer follow-up period are necessary to conclude whether serum levels of TGF-β1 carry significant clinical relevance. [See editorial on pages 517–9, this issue]. Cancer 1999;85:554–61. © 1999 American Cancer Society.