Background: There is paucity of information about bone metabolism during pregnancy or breast feeding in teenagers. Aim: To study bone turnover at the end of pregnancy and during breast feeding in teenagers and correlate it with environmental, hormonal or nutritional variables. Subjects and methods: Thirty teenagers during their breast feeding period after a first pregnancy and 30 nulliparous girls matched for age, age of menarche and body mass index were assessed three weeks after delivery (period 1), at six months of breast feeding (period 2) and one year after the lactating period (period 3). Calcium intake and plasma calcium, phosphorus, alkaline phosphatases, parathormone, estradiol and prolactin were measured. Calcium, creatinine and hydroxyproline were also measured in a morning urine sample. Results: Lactating and control girls were aged 16.3±0.8 and 16.1±0.7 years old respectively. Calcium intake in lactating and control girls was 798±421 and 640±346 g/day respectively in period 1, 612±352 and 592±309 mg/day in period 2 and 495±180 and 456±157 g/day in period 3. During periods 1 and 2, lactating girls had higher alkaline phosphatases (161±37 compared to 119±28 U/l and 149±37 compared to 106±23 U/l), parathormone (4.3±2.6 compared to 2.8±0.8 ng/dl and 3.6±1.6 compared to 3.0±0.9 ng/dl) and urinary hydroxyproline (95±16 compared to 63±15 mg/g creatinine and 84±19 compared to 59±15 mg/g creatinine). No differences were observed in period 3. No correlation between bone turnover variables, body mass index or hormonal parameters, was observed. Conclusions: In teenagers, there is an increase in bone turnover at the end of pregnancy, that persists during the lactating period. These changes are not related to nutritional or hormonal variables.