Clinician-guided internet-delivered cognitive behavioral therapy (iCBT) is an effective treatment for depression and anxiety disorders. However, few studies have examined the effectiveness of completely unguided iCBT. The current research investigated adherence to, and the effects of two brief unguided iCBT programs on depression and anxiety symptom severity, and psychological distress. Study 1 evaluated a four-lesson transdiagnostic iCBT program for anxiety and depression (N = 927). Study 2 then evaluated a three-lesson version of the same program (N = 5107) in order to determine whether reducing the duration of treatment would influence adherence and treatment effects. Cross-tabulations and independent t-tests were used to examine the extent to which users adhered and remitted with treatment. Linear mixed models were used to evaluate the effects of treatment in the entire sample, and stratified by gender and completer-type (e.g., users who completed some but not all lessons vs. those who completed all lessons of treatment). Among those who began treatment, 13.83% completed all four lessons in Study 1. Shortening the course to three lessons did not improve adherence (e.g., 13.11% in Study 2). In both studies, users, on average, experienced moderate to large effect size reductions in anxiety and depressive symptom severity, as well as psychological distress. This pattern of results was robust across gender and for those who did and did not complete treatment. Approximately two-thirds of those who completed treatment experienced remission. These data show that unguided iCBT programs, which have the capacity to attract large numbers of individuals with clinically significant symptoms of depression and anxiety, and psychological distress, can produce significant improvements in wellbeing.