Sixty adult patients (40 women, 20 men) with Marfan syndrome (MFS) according to the Berlin criteria had a full clinical examination and bone mineral density (BMD) measurement by dual-energy X-ray absorptiometry of the hip and nondominant forearm. BMD was expressed as a Z-score and compared with the reference population of the Hologic database. In MFS men, BMD (g/cm(2)) was compared with the BMD of 45 normal tall Caucasian adults. Osteocalcin was measured by radioimmunoassay. In patients with MFS, BMD was compared between patients with and without previous fractures and according to the phenotypic severity of MFS. The mean age of the patients was 32.9 +/- 9.3 years (women 32.5 +/- 9.7, men 33.4 +/- 8.6), mean height was 180.3 +/- 10.3 cm (women 176.3 +/- 9.2, men 188.1 +/- 7.5) and mean body mass index 20.9 +/- 3.6 kg/m(2) (women 20.8 +/- 3.4, men 20.95 +/- 3.97). Hyperlaxity score (Beighton criteria) was 6.9 +/- 1. 1. Six patients (10%) had a previous fracture. Thirty per cent of patients had had at least one previous operation for scoliosis, aortic dilatation or eye problems. BMD values in the 60 patients were as follows: Z-score of the hip, -1.26 +/- 0.93, p<10(-9) (neck, -0.93 +/- 1.09, p<10(-9); trochanter, -1.31 +/- 0.85, p<10(-9); intertrochanter, -1.39 +/- 0.99, p<10(-9); Ward's triangle, -0.93 +/- 1.88, p<10(-9)); Z-score of the radius: -1.6 +/- 1.06, p<10(-9) (1/3 proximal, -1.29 +/- 1.03; mid-radius, -1.94 +/- 1.04; ultradistal, -0.68 +/- 1.1, p<10(-9)). The decrease in BMD was similar in men and women at both the hip and the radius. BMD in MFS patients was significantly decreased at cortical compared with trabecular sites (radius 1/3 proximal vs ultradistal, p<0.0001; total femur vs Ward's triangle, p<0.0005). No difference in BMD was found between MFS patients with or without previous fractures and those with severe or less severe phenotypic expression of MFS. An influence of height and weight in MFS on BMD is suspected. Osteocalcin was not increased in our group of MFS patients. Thus both men and women with MFS have a significant deficit of BMD at the hip and radius. The decrease in BMD is present equally in both sexes and is more pronounced at predominantly cortical sites. In our group of patients we found no increase in fractures and no relation between decreased BMD and phenotypic expression of the syndrome.