This systematic review examines the adherence rates and dropout rates in randomized controlled trials (RCTs) of Internet interventions for anxiety and depression. The study found that dropout rates in RCTs were lower compared to open access websites, ranging from 1% to 50%. Adherence rates were relatively high, exceeding 50%. Predictors of adherence included disease severity, treatment length, and chronicity. However, few studies formally examined reasons for dropout, and most studies failed to use appropriate statistical techniques to analyze missing data. The review highlights the need for theoretical models of adherence in Internet intervention research and suggests that disease-based factors in anxiety and depression require further investigation. The findings also emphasize the importance of comparing adherence rates between open access interventions and RCTs to establish benchmarks.This systematic review examines the adherence rates and dropout rates in randomized controlled trials (RCTs) of Internet interventions for anxiety and depression. The study found that dropout rates in RCTs were lower compared to open access websites, ranging from 1% to 50%. Adherence rates were relatively high, exceeding 50%. Predictors of adherence included disease severity, treatment length, and chronicity. However, few studies formally examined reasons for dropout, and most studies failed to use appropriate statistical techniques to analyze missing data. The review highlights the need for theoretical models of adherence in Internet intervention research and suggests that disease-based factors in anxiety and depression require further investigation. The findings also emphasize the importance of comparing adherence rates between open access interventions and RCTs to establish benchmarks.