Migrainous disorder was analysed in a large population-based study of 4000 forty-year-old males and females. All interviews were conducted by one physician and the diagnostic criteria of the International Headache Society were used. Of the 48 people with migrainous disorder, 40 had migrainous disorder without aura and 9 had migrainous disorder with aura. One person had co-occurrence of migrainous disorder with and without aura. The lifetime prevalence of migrainous disorder was 2.5% with a male:female ratio of 1:1.2. The first-degree relatives of probands with migrainous disorder were blindly interviewed. Compared with the general population, first-degree relatives of probands with migrainous disorder without aura had a slightly but less increased risk of migraine without aura than first-degree relatives of probands with migraine without aura. First-degree relatives of probands with migrainous disorder with aura had no increased risk of migraine with aura. We conclude that migrainous disorder without aura in some people is a type of migraine without aura and in other people not. Migrainous disorder with aura may be unrelated to migraine with aura.