We examined 56 French Canadians, aged 1 week to 54 years, from eastern Quebec who were referred to the Laval University Lipid Research Centre and in whom coincidental finding (in 46% of the cases), abdominal pain (in 32%) or family screening (in 22%) led to the diagnosis of primary lipoprotein-lipase-activity deficiency (familial hyperchylomicronemia). Half of the patients had one or more of the following signs: lipemia retinalis, eruptive xanthomas, splenomegaly and hepatomegaly; the plasma triglyceride concentrations were significantly higher (greater than 40 mmol/L) among these patients than among those without clinical signs (mean 21.7 [standard deviation 13.5] mmol/L). The prevalence rate of this disorder was 30 times higher than the previously published rate and was highest in the counties of Charlevoix and Saguenay-Lac-St-Jean (200 and 100 cases per million respectively) because of the distinct demographic history of these areas. Because of a founder effect an autosomal recessive gene involved in lipoprotein-lipase expression or activation has probably been disseminated among this isolated French Canadian population.