OBJECTIVE--To study the effects of the introduction of electronic data interchange between primary and secondary care providers on speed of communication, efficiency of data handling, and satisfaction of general practitioners with communication. DESIGN--Comparison of traditional paper based communication for laboratory reports and admission-discharge reports between hospital and general practitioners and electronic data interchange. SETTING--Twenty-seven general practitioners whose offices were equipped with a practice information system and two general hospitals. OUTCOME MEASURES--Paper based communication was evaluated by questionnaire responses from and interviews with care providers; electronic communication was evaluated by measuring time intervals between generation and delivery of messages and by assessing doctors' satisfaction with electronic data interchange by questionnaire. RESULTS--Via paper mail admission-discharge reports took a median of 2-4 days, and laboratory reports 2 days, to reach general practitioners. With electronic data interchange almost all admission-discharge reports were available to general practitioners within one hour of generation. When samples were analysed on the day of collection (as was the case for 174/542 samples in one hospital and 443/854 in the other) the laboratory reports were also available to the general practitioner the same day via electronic data interchange. Fifteen general practitioners (of the 24 who returned the questionnaire) reported that the use of electronic admission-discharge reports provided more accurate and complete information about the care delivered to their patients. Ten general practitioners reported that electronic laboratory reports lessened the work of processing the data. CONCLUSION--Electronic communication between primary and secondary care providers is a feasible option for improving communication.