SummaryWe tested the effect of the Sleep Position Trainer, a vibrational device, for positional sleep apnea in an open, randomized controlled trial with 101 patients, where 52 patients were allocated to Sleep Position Trainer and 49 patients to a non‐treatment control group for 2 months (Part 1). All patients were then followed as a cohort for a period of 6 months with use of the Sleep Position Trainer (Part 2). The participants were assessed with polygraphy at entry, and after 2 and 6 months. The mean apnea–hypopnea index supine was 35 per h (SD, 18) in the Sleep Position Trainer group and 38 per h (SD, 15) in the control group at entry. In a per protocol analysis, the mean total apnea–hypopnea index at entry and after 2 months in the Sleep Position Trainer group was 18 per h (SD, 10) and 10 per h (SD, 9; P < 0.001) versus 20 per h (SD, 9) and 18 per h (SD, 10; NS) in the control group. The mean supine sleep time decreased from 47% (SD, 22) to 17% (SD, 18; P < 0.001) in the Sleep Position Trainer group after 2 months. In the control group, the mean supine sleep time was 48% (SD, 20) at entry and 39% (SD, 21; NS) after 2 months. The positive effect of Sleep Position Trainer was maintained in all patients treated with Sleep Position Trainer after 6 months. Daytime sleepiness improved after 6 months. Compliance with the Sleep Position Trainer device during the first 2 months, defined as use of Sleep Position Trainer >4 h per night for all weekdays, was 75.5% (SD 21.2). The discontinuation rate was 28.8 and 49.4% after 2 and 6 months, respectively.