We studied prospectively the acute and long-term compliance with nasal continuous positive airway pressure (nCPAP) therapy set up during a split-night polysomnography in 31 patients suffering from severe obstructive sleep apnea syndrome (OSAS). The mean apnea-hypopnea index (AHI) was 66 +/- 23/hour. An effective nCPAP (AHI < 10/hour in all sleep stages in the dorsal decubitus) was titrated in 27/31 patients. The mean effective nCPAP was 11 +/- 2 cm H2O. In three patients, a subsequent night was necessary to determine the effective nCPAP during rapid eye movement sleep, and one patient did not support the treatment. Of the 27 patients with successful titration, 21 accepted home treatment, three chose a surgical procedure and three refused to be treated. Of the 21 accepting home treatment, one patient did not receive his insurance agreement and could not participate in follow-up. Among the 20 other patients, four interrupted their treatment during the 1st month because of discomfort, and 16 were followed for 285 +/- 84 days. The daily rate of nCPAP use for the compliant patients was 6.7 +/- 1.5 hours. These preliminary results indicate that a split-night technique is reliable and cost saving in a majority of patients suffering from severe OSAS.