Background: Cobalamin deficiency is prevalent in vegetarians and has been associated with increased risk of osteoporosis. Aim of the study: To examine the association between cobalamin status and bone mineral density in adolescents formerly fed a macrobiotic diet and in their counterparts. Methods: In this cross-sectional study bone mineral density (BMD) and bone mineral content (BMC) were determined by DEXA in 73 adolescents (9-15 y) who were fed a macrobiotic diet up to the age of 6 years followed by a lacto-(-ovo-) vegetarian or omnivorous diet. Data from 94 adolescents having consumed an omnivorous diet throughout their lives were used as controls. Serum concentrations of cobalamin, methylmalonic acid (MMA) and homocysteine were measured and calcium intake was assessed by questionnaire. Analysis of covariance (MANCOVA) was performed to calculate adjusted means for vitamin B12 and MMA for low and normal BMC and BMD groups. Results: Serum cobalamin concentrations were significantly lower (geometric mean (GM) 246 pmol/L vs. 469 pmol/L) and MMA concentrations were significantly higher (GM 0.27 μmol/L vs. 0.16 μmol/L) in the formerly macrobiotic-fed adolescents compared to their counterparts. In the total study population, after adjusting for height, weight, bone area, percent lean body mass, age, puberty and calcium intake, serum MMA was significantly higher in subjects with a low BMD (p = 0.0003) than in subjects with a normal BMD. Vitamin B12 was significantly lower in the group with low BMD (p = 0.0035) or BMC (p = 0.0038) than in the group with normal BMD or BMC. When analyses were restricted to the group of formerly macrobiotic-fed adolescents, MMA concentration remained higher in the low BMD group compared to the normal BMD group. Conclusions: In adolescents, signs of an impaired cobalamin status, as judged by elevated concentrations of methylmalonic acid, were associated with low BMD. This was especially true in adolescents fed a macrobiotic diet during the first years of life, where cobalamin deficiency was more prominent. © Steinkopff Verlag 2004.