The effect of cobalt on the cardiovascular system is one of many aspects of cobalt metabolism in humans. Elastin and collagen are the main proteins of the vascular wall. The aims of this study were: 1) to determine serum cobalt concentrations in children with hypertension; and 2) to study the correlation between serum cobalt and some biological markers of the extracellular matrix of the vascular wall, i.e., anti-elastin and anti-collagen type IV antibodies. Patients showed statistically significant higher levels of systolic and diastolic blood pressure, and significantly lower serum cobalt concentrations, than controls. Children with hypertension showed significantly higher levels of total cholesterol (P = 0.0003) and collagen type IV IgM (P = 0.04). Collagen type IV IgG levels (P = 0.027) were lower than in controls. Serum cobalt in patients showed a correlation with systolic blood pressure (r = -0.44, P = 0.05), elastin IgM (r = 0.60, P = 0.007), and collagen type IV IgG (r = -0.46, P = 0.04). Our data suggest the existence of a correlation between changes in levels of serum cobalt, total cholesterol, anti-collagen type IV antibodies, and essential hypertension in children. This is the first study of serum cobalt in children with essential hypertension.