A consecutive series of 233 patients with persistent insomnia was studied with regard to personality traits by means of the Karolinska Scales of Personality (KSP). The group as a whole was found to have elevated scores on Psychasthenia, Muscular Tension, Somatic Anxiety, Psychic Anxiety, and Monotony Avoidance, and reduced scores on Socialization, Detachment, and Social Desirability. Patients with the subdiagnosis ''psychophysiological insomnia'' showed a relatively normal personality profile. Women over 50 years showed a special pattern with their most elevated score on Monotony Avoidance, and normal scores on Psychic and Somatic Anxiety. Analysis at the item level showed that, although Psychasthenia was the KSP scale that discriminated most strongly between insomniacs and controls, this does not indicate the presence of a genuinely psychasthenic personality; the insomniacs' elevated score on Psychasthenia was largely due to items that refer to typical symptoms of insomnia, like difficulty to regain lost sleep, fatigue, and concentration difficulties. The analysis suggested a number of possible factors that may be involved in the development of insomnia: self-imposed strain, excessive sensitivity, slow recuperation after stress, worrying, an overly serious attitude to life, negative family experiences, emotional over-involvement in others, dependence on others, and understimulation.