Quantitation of apneas and hypopneas is routinely included in studies of epidemiology, diagnosis, and treatment of sleep-disordered breathing (SDB). The definition of apnea appears clear-cut in the sleep literature. In contrast, the literature contains remarkable variety in both recording techniques and definitions of hypopnea. The purpose of this study was to characterize the variety in the definitions and techniques used to identify hypopnea in clinical sleep laboratories. One hundred surveys were mailed to 100 accredited sleep laboratories. Each laboratory was asked to provide its criteria and equipment used to define hypopnea. Forty-five surveys (45 percent) were returned. No two laboratories used the same definition and measures of hypopnea. We conclude that there is no consensus about either recording techniques or definitions of hypopnea. Thus, epidemiologic studies and reports of interventions on SDB that do not include precise definitions of hypopnea must be interpreted with caution.