BACKGROUND: Mindfulness has been shown to be effective in treating various medical and mental problems. Especially its incorporation in cognitive-behavioural interventions has improved long-term outcomes of those treatments. It has also been shown, that brief mindfulness-based trainings are effective in reducing distress. There have been few web-based interventions incorporating mindfulness techniques in their manual and it remains unclear whether a brief web-based mindfulness intervention is feasible. METHODS: Out of 50 adults (different distress levels; exclusion criteria: <18 years, indication of psychotic or suicidal ideation in screening) who were recruited via e-mail and screened online, 49 were randomized into an immediate 2-weeks-treatment group (N=28) or a waitlist-control group (N=21), starting with a 2-week delay. Distress (BSI), perceived stress (PSQ), mindfulness (FMI), as well as mood and emotion regulation (PANAS/SEK-27) were measured at pre-, post- and 3-month follow-up (3MFU). Intention-to-treat analyses using MI for missing data and per-protocol analyses (≥50% attendance) were performed. RESULTS: 26 participants of the treatment group completed post-measures. Most measures under ITT-analysis revealed no significant improvement for the treatment group, but trends with medium effect sizes for PSQ (d=0.46) and PANASneg (d=0.50) and a small, non-significant effect for FMI (d=0.29). Per-protocol analyses for persons who participated over 50% of the time revealed significant treatment effects for PSQ (d=0.72) and PANASneg (d=0.77). Comparing higher distressed participants with lower distressed participants, highly distressed participants seemed to profit more of the training in terms of distress reduction (GSI, d=0.85). Real change (RCI) occurred for PSQ in the treatment condition (OR=9). Results also suggest that participants continued to benefit from the training at 3MFU. CONCLUSION: This study of a brief web-based mindfulness training indicates that mindfulness can be taught online and may improve distress, perceived stress and negative affect for regular users. Although there were no significant improvements, but trends, for most measures under ITT, feasibility of such a program was demonstrated and also that persons continued to use techniques of the training in daily life. TRIAL REGISTRATION: German Clinical Trials Register (DRKS): DRKS00003209.