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Markers of bone turnover are often elevated in patients with prevalent bone metastases (BM). To test whether bone markers may be used as early indicators of developing BM, we prospectively studied 113 women with primary breast cancer. At the time of study inclusion, none of the women had BM, skeletal disease or was on bone active drugs. During follow-up (8-52, median 30 mo.), pt. were seen every 3 mo. and blood/urine specimens were obtained. Eleven patients developed BM (BM+) and each of them was matched to 4 women remaining free of BM (BM-). Markers were serum (s) calcium, sTAP, sBAP, sOC, sPICP, sNTX, sCTX and urinary (u) PYD, uDPD, uNTX, uCTX. All analyses were done in single batches after study end. At any given point in time, marker levels in the BM+ group did not differ from those in the BM- group. Levels at baseline did not predict later BM (OR 0.14-1.01, all ns). 93% of all changes in bone markers were below the least significant change, as defined in an independent group of similar patients. The remaining 7% of values could not be associated in a consistent pattern with the occurrence of BM. We conclude that in patients with primary breast cancer, biochemical markers of bone turnover can not be used to predict or diagnose incident BM. This lack in diagnostic validity is mainly attributable to the high overall and long-term variability of the currently used bone markers.