Transforming growth factor β (TGF-β) plays an important role in cancer. Monoclonal antibodies (mAb) designed to specifically block the TGF-β ligands, are expected to inhibit tumor progression in patients with metastatic cancer. TβM1 is a humanized mAb optimized for neutralizing activity against TGF-β1. The objective of this clinical trial was to assess the safety and tolerability of TβM1 in patients with metastatic cancer. In this phase I, uncontrolled, non-randomized, dose-escalation study, 18 eligible adult patients who had measurable disease per RECIST and a performance status of ≤ 2 on the ECOG scale were administered TβM1 intravenously over 10 min at doses of 20, 60, 120 and 240 mg on day 1 of each 28-day cycle. Safety was assessed by adverse events (as defined by CTCAE version 3.0) and possible relationship to study drug, dose-limiting toxicities and laboratory changes. Systemic drug exposure and pharmacodynamic (PD) parameters were assessed. TβM1 was safe when administered once monthly. The pharmacokinetic (PK) profile was consistent with a mAb with a mean elimination half-life approximately 9 days. Although anticipated changes in PD markers such as serum VEGF, bFGF and mRNA expression of SMAD7 were observed in whole-blood, suggesting activity of TβM1 on the targeted pathway, these changes were not consistent to represent a PD effect. Additionally, despite the presence of an activated TGF-β1 expression signature in patients' whole blood, the short dosing duration did not translate into significant antitumor effect in the small number of patients investigated in this study.