Introduction. The EORTC 22922/10925 trial investigated the potential survival benefit and toxicity of elective irradiation of the internal mammary and medial supraclavicular (IM-MS) nodes Accrual completed in January 2004 and first results are expected in 2012. We present the toxicity reported until year 3 after treatment. Patients and methods. At each visit, toxicity was reported but severity was not graded routinely. Toxicity rates and performance status (PS) changes at three years were compared by χ² tests and logistic regression models in all the 3 866 of 4 004 patients eligible to the trial who received the allocated treatment. Results. Only lung (fibrosis; dyspnoea; pneumonitis; any lung toxicities) (4.3% vs. 1.3%; p < 0.0001) but not cardiac toxicity (0.3% vs. 0.4%; p = 0.55) significantly increased with IM-MS treatment. No significant worsening of the PS was observed (p = 0.79), suggesting that treatment-related toxicity does not impair patient's daily activities. Conclusions. IM-MS irradiation seems well tolerated and does not significantly impair WHO PS at three years. A follow-up period of at least 10 years is needed to determine whether cardiac toxicity is increased after radiotherapy.