Between March 1 and Sept 1, 1980, we interviewed a 25% random sample of patients admitted to medicine, surgery, and obstetric, and gynecology services to determine the frequency and descriptive characteristics of night sweats (NS). Seventy-two (41%) of 174 patients interviewed reported NS within three months before admission. Obstetric patients reported NS significantly more often than nonobstetric patients (60% vs 33%, P less than .02). The duration of NS ranged from one day to 27 years (mean 10.5 months; median two months). NS were mild in 36 (50%), moderate in 17 (24%), and severe in 19 (26%). Severe NS were reported significantly more often by nonobstetric patients, and most often by those on the medicine service. Among nonobstetric patients, NS were associated with metastatic adenocarcinoma of the prostate gland, and severe NS with the use of antipyretics. NS were not associated with elevated temperature measurements during hospitalization.