Plasma fibrinogen levels were measured in 1,175 Israeli schoolchildren, aged 9-18 years, originating from diverse ethnic groups. The Moslem children displayed striking differences in levels and age-patterns of fibrinogen compared to Jewish children. Lower mean plasma fibrinogen levels in boys aged 9-10, 13-14 and 16-18 were observed among Moslem boys, compared to their Jewish counterparts born in Israel. Moslem girls showed lower levels of fibrinogen than Jewish Israeli girls at ages 9-10 and 16-18. While the Jewish children displayed an age-associated fibrinogen pattern comparable at the three age groups, the levels for Moslem children increased pre-puberty and decreased post-puberty, peaking at 13-14 years. No significant difference in mean plasma fibrinogen was found between sexes within ethnic groups. A number of statistically significant but low correlations (-0.32 to 0.24) were found between plasma fibrinogen and high density lipoprotein cholesterol (HDL-C) as a percentage of total cholesterol, triglycerides, blood glucose, uric acid, blood pressure, Quetelet index and sports activity. The difference between fibrinogen levels might point to a possible ethnicity risk factor explanation rather than environmentally acquired factors.