Clinicians use undefined medical jargon in written communication with patients, which can undermine understanding and satisfaction. Very little is known about educational interventions to reduce clinicians’ use of undefined jargon in written communication with patients. We retrospectively analyzed jargon in 421 messages sent by medical residents to patients through a patient portal during the 3 months before and 3 months after a workshop on clear written communication. Undefined jargon decreased from 7.1 terms per 100 words to 6.3 terms per 100 words (p<.0001). Residents were not aware of the study at the time, indicating the development of plain language “habits.”