The present study was carried out to evaluate the levels of plasma fibronectin (Fn) in cancer patients undergoing radiation therapy (RT) in correlation with outcomes in terms of radiation toxicity. A total of 26 patients with lung and gastrointestinal (GI) cancer, treated with RT were enrolled in this study. Plasma Fn levels were determined before and following a course of RT. The Radiation Therapy Oncology Group (RTOG) criteria were used to determine the grade of RT toxicity. Statistical analysis utilised the nonparametric Mann–Whitney U-test as well as bivariate linear regression. Pre-RT Fn levels were significantly higher in cancer patients without toxicity (median ± SE) (485.0 ± 87 μg/ml) as compared with the levels of plasma Fn in patients with grade I–II RTOG acute toxicity (354.0 ± 74 μg/ml, p = 0.01). No significant difference in Fn levels was found in patients with grade I toxicity compared with patients with grade II toxicity. In addition, low baseline Fn levels (148 and 299 μg/ml) were observed in two lung cancer patients who developed symptomatic pneumonitis during the first 2 months after RT. These preliminary results suggest that low baseline Fn may have potential as a predictive marker for development of RT-induced toxicity.