Worry is an important mediator in the relation between stressors and health. This pilot study examined whether a smartphone-based in time worry-reduction training was feasible and improved physiological health (i.e. increased heart rate variability [HRV]). A total of 26 high-worriers were randomised to an experimental or active-control condition (EC and CC respectively). Participants in both conditions registered emotions 5 times daily for a month. The EC additionally received worry-reduction training with mindfulness exercises. Primary outcomes were feasibility and HRV measured at baseline, after 2 weeks (halfway), and at 4 weeks (post-intervention). Both training conditions were feasible and well received. HRV increased in the EC and CC, but this increase did not differ between conditions. Preliminary findings suggest that both training conditions are feasible and might improve HRV, which is an important predictor of cardiovascular disease. This pilot study only provided preliminary evidence, but it laid the groundwork for future randomised controlled trials that ought to include more participants and a waitlist control group in order to get more definitive evidence of the effectiveness of the intervention.