In patients with chronic respiratory failure (CRF) noninvasive mechanical ventilation (NMV) improves quality of life. We studied some basic issues concerning sexuality in patients with NMV. In 383 patients with NMV for CRF (age, > 40 yr) physiologic data (lung function, blood gases, and exercise) were taken from within the 6 mo period before enrollment. The questionnaire was focused on sexuality after initiation of NMV. Of the patients, 54.3% sent back the questionnaire. NMV was used for 41.1 +/- 27.0 mo. A total of 34.1% of patients were sexually active. Compared with patients receiving NMV, control persons had a higher rate of sexual activity (84%, p < 0.0001) and masturbation rate (13 versus 40%). Sexually active patients had greater VC (2.1 versus 1.8 L), higher FEV(1) (1.4 versus 1.1 L), higher Pa(O(2)) at rest (64.0 versus 60.4 mm Hg), a higher maximal work load (72.0 versus 58.8 W), were younger, and most of them were married or had sexual partners. Changes in sexual activity after NMV initiation were reported to be as follows: "Nothing changed," 46.3%; "less active," 35.8%; "more active," 12.6%; and "fantasy increased," 10.5%. Increased sexual fantasy predominated in men. "Sexually active" patients with NMV had sexual intercourse 5.4 +/- 4.8 times per month. Sexuality in patients receiving NMV for CRF is markedly reduced compared with normal subjects. In half of the patients, sexual activity is influenced by initiation of NMV.