Trust in the oncologist is crucial for breast cancer patients. It reduces worry, enhances decision making, and stimulates adherence. Optimal nonverbal communication by the oncologist, particularly eye contact, body posture, and smiling, presumably benefits patients' trust. We were the first to experimentally examine (1) how the oncologist's nonverbal behavior influences trust, and (2) individual differences in breast cancer patients' trust. Analogue patients (APs) viewed one out of eight versions of a video vignette displaying a consultation about chemotherapy treatment. All eight versions varied only in the oncologist's amount of eye contact (consistent vs. inconsistent), body posture (forward leaning vs. varying), and smiling (occasional smiling vs. no smiling). Primary outcome was trust in the observed oncologist (Trust in Oncologist Scale). 214 APs participated. Consistent eye contact led to stronger trust (β = -.13, p = .04). This effect was largely explained by lower educated patients, for whom the effect of consistent eye contact was stronger than for higher educated patients (β = .18, p = .01). A forward leaning body posture did not influence trust, nor did smiling. However, if the oncologist smiled more, he was perceived as more friendly (r s = .31, p < .001) and caring (r s = .18, p = .01). Older (β = .17, p = .01) and lower educated APs (β = -.25, p < .001) were more trusting. Trust was weaker for more avoidantly attached APs (β = -.16, p = .03). We experimentally demonstrated the importance of maintaining consistent eye contact for breast cancer patients' trust, especially among lower educated patients. These findings need to be translated into training for oncologists in how to optimize their nonverbal communication with breast cancer patients while simultaneously managing increased time pressure and computer use during the consultation.