Of 26,294 hospitalized medical patients monitored in a drug surveillance program, 366 (1.4%) received meperidine orally and 3268 (12.4%) received meperidine parenterally during one or more admissions. Neoplastic disease was the most common category of primary diagnosis (43%) among oral recipients; among parenteral recipients cardiovascular and neoplastic diseases (23% each) were most common. Oral meperidine was prescribed almost exclusively (93%) for pain relief, whereas the parenteral drug also had substantial use (41%) as a diagnostic and preoperative adjunct. Oral meperidine was judged by attending physicians to have provided unsatisfactory pain relief in 22% of recipients, while parenteral meperidine was rated unsatisfactory in 11%. The drug was less effective in patients with a primary diagnosis of neoplastic disease than in patients with other primary diagnoses. Minor gastrointestinal disturbances were the most commonly reported adverse reactions among oral meperidine recipients (2.7%); among parenteral recipients central nervous system effects were most common (1.2%). Seven recipients had life-threatening adverse reactions. However, only two of these were judged definitely related to meperidine, and in both instances other drugs were also definitely implicated. Adverse reactions were dose related.