BACKGROUND: E-health services may improve the quality and efficiency of care; however, there is little quantitative data on e-health use. OBJECTIVE: The objective of this study was to examine trends in e-health use and user characteristics. RESEARCH DESIGN: This was a longitudinal study of e-health use (1999-2002) within an integrated delivery system (IDS). We classified 4 e-health services into transactional (drug refills and appointment scheduling) and care-related (medical and medication advice) services. SUBJECTS: Approximately 3.3 million members of a large, prepaid IDS. MEASUREMENTS: Amount and frequency of e-health use over time and characteristics of users. RESULTS: The number of members registered for access to e-health increased from 20,617 (0.7% of all members) in Q1 1999 to 270,987 (8.6%) in Q3 2002. Between Q1 and Q3 2002, 42,845 members (1.3%) used the drug refill service and 55,901 (1.7%) used the appointment scheduling service compared with 10,756 members (0.3%) who used the medical advice service and 3069 (0.1%) who used the medication advice service. Over the same period, transactional service users averaged 3.5 uses/user versus 1.6 uses/user among care-related service users. Members most likely to use e-health services had a high level of clinical need, a regular primary care provider, were 30 to 64 years old, female, white, and lived in a nonlow socioeconomic status neighborhood. These findings were consistent across e-health service types. CONCLUSIONS: Although use of all e-health services grew rapidly, use of care-related services lagged significantly behind use of transactional services. Subjects with greater clinical need and better ties to the health system were more likely to use both types of e-health services.