Transforming growth factor-beta (TGF β)1 is thought to be implicated in breast cancer progression. However, data about the influence of TGF β1 on breast cancer development are conflicting. To clarify the clinical relevance of TGF β1, TGF β1 protein level has been measured by enzyme-immoassay in 193 breast tumour samples. We found that 94.3% of patients expressed TGF β1 with a range of 0–684 pg mg −1 protein. In the overall population, an increase of tumoral TGF β1 was observed in premenopausal patients when compared to postmenopausal subgroup ( P=0.0006). When patients were subdivided according to nodal status, TGF β1 was correlated to type-1 plasminogen activator inhibitor in the node-negative subgroup ( P=0.040). Multivariate analysis revealed that, after lymph node status ( P=0.0002) and urokinase-type plasminogen activator ( P=0.004), TGF β1 was an independent prognostic marker for DFS ( P=0.005) in the overall population. In the node-negative population, TGF β1 was the prominent prognostic factor ( P=0.010). In the same population, Kaplan–Meier curves demonstrated that high TGF β1 level was correlated with a shorter disease-free survival ( P=0.020). These data suggest that the measurement of tumoral TGF β1 protein level, especially for node-negative patients, might help to identify a high-risk population early in tumour progression.