The occurrence of reduced bone mineral density (BMD) among adolescent girls with oligomenorrhea or secondary amenorrhea, due to 'pure' dysfunction of the hypothalamo-pituitary-ovarian (HPO) axis (without anorexia nervosa, excessive sport or ballet, slimming diet, etc.) was examined. The study group consisted of 19 adolescent girls (age 16-18 years) with oligo/amenorrhea. Clinical (height, weight, age at menarche, duration of amenorrhea, body mass index (BMI)), hormonal (follicle stimulating hormone (FSH), luteinizing hormone (LH), estradiol, progesterone, testosterone, prolactin), and ion (calcium, sodium, potassium, phosphate, chloride) parameters and the BMD of the lumbar spine were investigated. Correlations between BMD and other parameters were also examined. Twenty healthy volunteers (same age and regular cycles) served as controls. Three girls had osteoporosis, with a BMD below -2 standard deviations (SD). Ten showed osteopenia, with a BMD value between -1 and -2 SD. Only six of the study group had a normal BMD within +/- 1 SD. A positive correlation was observed between the BMD and the BMI (r = 0.73; p < 0.05). All the controls had normal hormonal, ion and BMD parameters. 'Pure' dysfunction of the HPO axis in adolescents, causing oligomenorrhea or secondary amenorrhea, might result in reduced BMD and, consequently, lower peak bone mass. Treatment of menstrual cycle disorders is necessary for the prevention of osteoporosis.