Biochemical markers were measured to assess bone turnover in a cross-sectional study of 43 patients with anorexia nervosa; 28 were at their first assessment (untreated) with a body mass index (BMI) (median interquartile range) of 13.3 (2) kg/m2. A second group of 15 patients undergoing treatment (treated) had a median BMI of 17.6 (2.8) kg/m2. The median, interquartile range of urinary deoxypyridinoline (DPyd), a bone resorption marker, was raised in both groups compared with an age-matched control population [DPyd = 17.8 (15.2), 17.5 (16.4) and 9.2 (4.0) nmol/mmol creatinine, respectively]. Serum type 1 collagen carboxyterminal propeptide (P1CP), a marker of bone formation, was similar to controls in the untreated patients [112 (29) and 112 (78.5) ng/ml, respectively], but was significantly raised in the treated patients [163 (219) ng/ml, P < 0.05]. A second group of 21 patients was followed prospectively, on admission and during 8 weeks of intensive inpatient care (BMI on admission and after 8 weeks was 13.0 (2) and 16.7 (3) kg/m2, respectively). The resorption marker, serum type 1 collagen carboxyterminal telopeptide (1CTP) was raised on admission and remained high during treatment. P1CP and osteocalcin levels were similar to control levels on admission but increased with treatment, and after 8 weeks were 40% and 63% higher respectively than on admission. These findings suggest that in untreated anorexia nervosa there was uncoupling of bone turnover as bone resorption markers were raised without a concomitant increase in bone formation markers. As the condition was treated and patients gained weight, the formation markers also increased, leading to a more balanced, although higher, bone turnover.