In a survey of cardiovascular risk factors in 185 men and 173 women of a Belgian population group, an independent and highly significant positive correlation was found between the serum concentrations of 25-hydroxyvitamin D3 and apolipoprotein A-I (p less than 0.001 in both sexes). 25-Hydroxyvitamin D3 also showed a positive correlation with high density lipoprotein cholesterol levels (p less than 0.05 in men and p less than 0.005 in women). This relation was independent of a possible effect of 25-hydroxyvitamin D3 on serum calcium. Taking the biological variation of 25-hydroxyvitamin D3 levels (+/- 2 SD = 25 ng/ml) into account, these findings could explain variations in the levels of apolipoprotein A-I of 15 mg/100 ml and of high density lipoprotein cholesterol of 4 mg/100 ml.