We conducted a prospective observational study to (1) determine usage and construct validity of a method to gauge the cognitive impact of information derived from daily e-mail, and (2) describe self-reported impacts of research-based synopses (InfoPOEMs) delivered as e-mail. Ratings of InfoPOEMs using an Impact assessment scale provided (a) data on usage of the impact assessment method, (b) reports of impact by InfoPOEM and by doctor and (c) data for analysis of construct validity of the scale. PARTICIPANTS were family physicians or general practitioners who rated at least five InfoPOEMs delivered on e-mail. For each InfoPOEM rated, 0.1 continuing education credit was awarded by the College of Family Physicians of Canada. Use of the impact assessment scale linked to a daily InfoPOEM was sustained during the 150-day study period. 1,007 participants submitted 61,493 reports of 'cognitive impact' by rating on average 61 InfoPOEMs (range 5-111). 'I learned something new' was most frequently reported. 'I was frustrated as there was not enough information or nothing useful' was the most frequently reported negative type of impact. The proportion of reports of 'No Impact' varied substantially across individual InfoPOEMs. Impact patterns suggested an 8 or 9-factor solution. Our Impact assessment method facilitates knowledge transfer by promoting two-way exchange between providers of health information and family doctors. Providers of health information can use this method to better understand the impact of research-based synopses. Sustaining current practice and increasing knowledge about new developments in medicine are important outcomes arising from research-based synopses delivered as e-mail, in addition to practice change.