We investigated whether a short course in communication skills for physicians would improve the quality of informed consent in a randomized clinical adjuvant trial on breast cancer. In this prospective, case-controlled intervention study, physicians and research nurses who introduced the cancer treatment trial to patients at three of the participating hospitals first attended a one-day communication skills course. The quality of informed consent was then evaluated by addressing a standardized questionnaire, QuIC, to trial patients at the three intervention hospitals and at control hospitals. Response rate was 90.0% (n = 288). Of the patients treated by the intervention group, 73% were very satisfied with the information received compared with 56% of those of the control group (p = 0.003). The patients of the intervention group considered the time given for making their decision sufficient more often than those of the controls (98% vs. 90%, p = 0.004). The patients of the intervention group recalled more often than those of the controls that the physician had also offered other therapeutic options than the trial treatment (91% vs. 97%, p = 0.032). They also understood the main aim of the study better than the patients of the controls (89% vs. 78%, p = 0.030). In conclusion, a short communication skills course for the trial physicians and nurses improved the quality of informed consent and patient satisfaction in the trial.