Probable migraine (PM) is a prevalent migraine subtype fulfilling all but one criterion for migraine with or without aura. The aims of this study were: (i) to describe the epidemiology, medical recognition and patterns of treatment for PM in the USA; (ii) to compare the patterns of preventive PM treatment in the population with expert panel guidelines for preventive treatment. A validated self-administered headache questionnaire was mailed to a random sample of 120,000 US households. Subjects were classified as PM according to the second edition of the International Classification of Headache Disorders (ICHD-2). The questionnaire also assessed patterns of migraine treatment. Guidelines for preventive medication use were developed by a panel of headache experts, who used headache frequency and impairment to assess the need for preventive therapy and the gap between current and ideal use. Our sample consisted of 162 576 individuals aged > or = 12 years. The 1-year period prevalence of PM was 4.5% (3.9% in men and 5.1% in women). In women and men, prevalence was higher in middle life, between the ages of 30 and 59 years. The prevalence of PM was significantly higher in African-Americans than in Whites (female 7.4% vs. 4.8%; male 4.8% vs. 3.7%) and inversely related to household income. During their headaches, most (48.2%) had at least some impairment, while 22.1% were severely disabled. The vast majority (97%) of PM sufferers used acute treatments, although 71% usually treated with over-the-counter medication. Most PM sufferers (52.8%) never used a migraine-preventive treatment and only 7.9% were currently using preventive medication. According to the expert panel guidelines, prevention should be offered (16.9%) or considered (11.5%) for 28.4% of the PM sufferers in the survey. We conclude that PM is a frequent, undertreated, sometimes disabling disorder. It has an epidemiological profile similar to migraine. In contrast to migraine, which is less prevalent in African-Americans than in Whites, PM is more prevalent in African-Americans than in Whites. In the USA, many with PM do not receive adequate treatment.