The Internet is a promising channel for delivering health-promoting interventions. A common problem for Internet-based interventions is low adherence. The current paper reports adherence rates from three different Internet-based trials with potential covariates.Data on adherence and baseline characteristics of users were collected from three different Internet-based trials: one supporting diabetes self-management, one supporting smoking cessation, and one offering an online personal health record. Logging of web use was used as the measure of adherence in two of the trials, while logging of authentication SMS messages was used in the third.In all three trials, users dropped out at a high rate early in the intervention. The baseline variables that were related to use were self-efficacy, having smoking friends, age, gender, and education. Tailored emails increased use for up to five months into a one-year intervention.Dropout from Internet-based trials is substantial, and attrition curves can be a valuable tool for more accurate pretrial estimates of sample size power. Automated follow-up of users via email seems likely to increase adherence and should be included in Internet-based interventions. Tailoring on baseline covariates to adherence such as self-efficacy could make them even more effective.