Twenty-five adult subjects with severe morbid onychophagia (nail biting) and no history of obsessive-compulsive disorder were enrolled in a 10-week double-blind cross-over trial of clomipramine hydrochloride and desipramine hydrochloride. For the 14 subjects who completed the study, clomipramine hydrochloride (mean +/- SD dose, 120 +/- 48 mg/d) was superior to desipramine hydrochloride (mean +/- SD dose, 135 +/- 53 mg/d) in decreasing nail biting as measured by a repeated-measures analysis of variance on the Nail Biting Severity, Nailbiting Impairment, and Clinical Progress scales. The high dropout rate at every stage of the study was in sharp contrast to that seen with psychiatric populations. From a neuroethologic perspective, similar biologic systems are hypothesized to mediate a spectrum of grooming behaviors, including onychophagia, trichotillomania, and obsessive-compulsive disorder.