Epidemiological studies of insomnia in the general population have reported high prevalence rates. However, few have applied diagnostic criteria from existing classification systems. Consequently. It is not possible to determine whether subjects suffered from a sleep disorder or whether the insomnia constituted a symptom of a mental disorder. Insomnia and its relationship with other mental disorders was investigated in the general population using DSM-IV criteria. A representative sample of 5622 subjects from the French population were interviewed about their sleep habits over the telephone by lay interviewers. The course and content of interviews were customized by means of the Sleep-Eval knowledge-based system. A total of 18.6% of the sample reported insomnia complaints. The presence of insomnia complaints, lasting for at least one month with daytime repercussions was found for 12.7% of the sample. Subsequently, subjects were classified according to the Sleep Disorder decision-making process proposed by the DSM-IV classification, but without the recourse of polysomnographic recordings. Specific sleep disorder diagnoses were given for 5.6% of the sample, mostly as insomnia related to another mental disorder, primary insomnia was given for 1.3% of the sample. Primary mental disorder diagnoses were supplied for 8.4% of the sample, mostly as generalized anxiety disorder. The results of this investigation emphasize the need to use classifications to determine whether subjects with insomnia complaints suffer from a sleep disorder or whether insomnia constitutes a symptom of some other mental disorder. These distinctions are of utmost importance as they have a bearing on the choice of treatment. Conversely, diagnoses were obtained by lay interviews, which may have caused a lack of recognition and/or discrimination for light or borderline symptomatology.