Data from three consecutive nights in the sleep laboratory and data from psychological questionnaires and interviews were cluster-analyzed. A total of 89 physician-referred insomniacs were included, together with 10 good sleepers. The goal was to develop a purely empirical classification scheme of insomnia and to compare it with the Association of Sleep Disorders Centers' current nosology, which is based on clinical experience. The categories of chronic psychophysiologic insomnia and of childhood-onset insomnia were clearly identified in the cluster analysis. The insomnias associated with psychiatric disorders were split into five subgroups that made some intuitive sense, but did not mesh exactly with DSM III (Diagnostic and Statistical Manual, 3rd edition, American Psychiatric Association) categories.