We sought to study the etiology of and risk factors for cerebral infarction in young adults in Hordaland County, Norway. All patients aged 15-49 years living in Hordaland County with a first-ever cerebral infarction during 1988-97 were included. Etiology was analyzed in subgroups defined by sex, age (<40 years versus >/=40 years), circulation territory (anterior versus posterior circulation) and short-term functional outcome [modified Rankin score (mRS) </= 2 versus mRS > 2]. A questionnaire was used to evaluate possible risk factors amongst the patients compared with an age- and sex-matched control group. The distribution of etiology was significantly different in all subgroups. Atherosclerosis was frequent amongst men (22.8% vs. 4.2%) and patients >/= 40 years (20.8% vs. 2.7%). All patients with microangiopathy had favorable short-term outcome. Significant risk factors were smoking more than 15 cigarettes per day (P < 0.001), hypertension (P = 0.001), and myocardial infarction (P = 0.035). Modifiable risk factors were frequent.