Our purpose was to compare the effectiveness and side effects of a novel, single-piece mandibular advancement device (OSA-Monobloc) for sleep apnea therapy with those of a two-piece appliance with lateral Herbst attachments (OSA-Herbst) as used in previous studies. An OSA-Monobloc and an OSA-Herbst with equal protrusion were fitted in 24 obstructive sleep apnea patients unable to use continuous positive airway pressure (CPAP) therapy. After an adaptation period of 156 +/- 14 d (mean +/- SE), patients used the OSA-Monobloc, the OSA-Herbst, and no appliance in random order, using each appliance for 1 wk. Symptom scores were recorded and sleep studies were done at the end of each week. Several symptom scores were significantly improved with both appliances, but to a greater degree with the OSA-Monobloc. Epworth Sleepiness Scale scores were 8.8 +/- 0.7 with the OSA-Herbst, and 8.6 +/- 0.8 with the OSA-Monobloc devices, and 13.1 +/- 0.9 without therapy (p < 0.05 versus both appliances). The apnea/hypopnea index was 8.7 +/- 1.5/h with the OSA-Herbst and 7.9 +/- 1.6/h with the OSA-Monobloc device, and 22.6 +/- 3.1/h without therapy (p < 0.05 versus both appliances). Side effects were mild and of equal prevalence with both appliances. Fifteen patients preferred the OSA-Monobloc, eight patients had no preference, and one patient preferred the OSA-Herbst device (p < 0.008 versus OSA-Monobloc). We conclude that both the OSA-Herbst and the OSA-Monobloc are effective therapeutic devices for sleep apnea. The OSA-Monobloc relieved symptoms to a greater extent than the OSA-Herbst, and was preferred by the majority of patients on the basis of its simple application.