Four patients between 15 and 25 years of age presented with exclusively expiratory groaning during sleep. Groaning usually occurred during the second part of the night, beginning at age 5 to 16 years. Patients were unaware of the nocturnal noise, but it alarmed others. Results of otorhinolaryngologic and neurologic examinations were normal. Expiratory groaning arose during REM and non-REM sleep stage 2, and was repeated in clusters. Nocturnal groaning, which the authors term catathrenia, represents a distinctive parasomnia.