Individuals with Down syndrome (DS) have a high prevalence of obesity and low bone mineral density (BMD), but body composition assessment needs additional study in DS. Three studies examined the agreement between body fat percentage (BF%) from air displacement plethysmography (ADP) and dual-energy x-ray absorptiometry (DXA), BF% from a four-compartment (4C) model and skinfolds (SF) and bioelectrical impedance analysis (BIA), and bone mineral content (BMC) from DXA and BIA. Sixty-six adults participated (33 DS, 33 control). In the first study, DXA provided higher BF% than ADP in both DS (t = 5.252, df = 32, p < .000) and controls (t = 7.714, df = 32, p < .000). In the second study, BF% from four BIA equations was not significantly different from 4CBF% in DS (p > .01), but these had a standard error of estimate (SEE) from 4.8 to 6.0 and wide limits of agreement (± 9.5% to ± 11.6%). Two BIA equations were not significantly different than 4CBF% in controls (p < .01), but these had SEE’s of 7.2 and 7.8 and wide limits of agreement (± 24.6% and ± 22.9%). Two SF equations were not significantly different from 4CBF% in DS (p < .007). Two of the SF equations were significantly different from 4CBF% in controls (p < .008). In the third study, BMC from DXA was significantly lower than BIA in DS (t = -5.237, df = 20, p < .000). DXA was significantly higher in controls (t = 3.110, df = 20, p = .006). There was no significant difference in DS males (t = -1.116, df = 7, p = .301) or control females (t = -7.978, df = 12, p = .000). DXA was significantly higher in control males (t = 5.641, df = 7, p = .001) and significantly lower in females with DS (t = -7.978, df = 12, p = .000). In conclusion, many methods of BF% assessment may be acceptable for adults with DS, but only appropriate equations should be selected. BIA should not currently be used to assess BMC in DS.