The objective of this study was to compare the bone mineral density (BMD) of men with Down syndrome (DS) to otherwise mentally retarded (MR) men and to investigate whether leg muscle strength of these patients is related to BMD. Two groups with MR (with and without DS) participated in the study, having met the following criteria: similar age, moderate to mild mental retardation, Tanner stage V of sexual development, similar age of beginning to walk, and equal motor activities. The DS group consisted of 8 men 23.9 +/- 4.2 years, and the MR group without DS consisted of 8 men 23.5 +/- 3.6 years. The two groups were compared with 10 sedentary students of the same age range (25.9 +/- 2.9 years) attending our University. The BMD of the 2(nd) to 4(th) lumbar vertebrae was measured in the PA projection and the mean density was expressed as g/cm(2). The isokinetic muscle strength of the right quadriceps femoris and hamstrings muscles was measured on a Cybex II isokinetic dynamometer. The value measured was peak torque at angular velocities at 60, 120, and 300 degrees.sec(-1). The results showed that BMD in DS individuals versus young adults (reference group of the scanner) was lower at the 26% level (T-score - 2.66 +/- 0.29) and significantly lower (P = 0.002) than that of the MR group. Significantly different muscle strength was observed between the DS and non-DS MR group (in quadriceps at 300 degrees.s(-1): P < 0.01, at 120 and 60 degrees. s(-1): P < 0.05; in hamstrings at 300 degrees.s(-1): P < 0.05). Higher differences in muscle strength were found between MR and control men, but no significant difference existed in BMD between them. Bivariate correlation showed that quadriceps strength significantly predicted the BMD in the DS patients. Active lifestyle and increased physical exercise to improve muscular strength should be instituted to avoid the development of osteoporosis in DS patients.