In adult cancellous bone, cyclic remodeling occurs in discrete locations (BMUs, or basic multicellular units). Some aspects of these remodeling cycles are still not clear. Only about half the osteoid surface normally has tetracycline labels; the remainder is thought to be immature or in a resting phase. This study was designed to test the assumption that pauses occur during the bone formation periods. Iliac crest bone biopsies were performed in 84 women with postmenopausal osteoporosis. The subjects were given three tetracycline labels; the middle label had a distinctive color. The osteoid surface was 10 +/- 6% of the bone surface; the range was 0.6-24.4%. The mineralizing surface was 42 +/- 26% of the osteoid surface. Immature osteoid was 7.6 +/- 5.3% of the osteoid surface. There were no BMUs with the first and third tetracycline label but without the middle demeclocycline label. The expected length of double 1-3 label, based on an assumption that unlabeled mature osteoid was temporarily in a resting phase, was significantly higher than the observed length (68 +/- 77 microns/mm2 tissue area versus 0, p < 0.0001). The mean active formation period was 77 +/- 16 days calculated by standard methods but was only 55 +/- 26 days when calculated by label escape (p < 0.001). The correlation coefficient between these two methods was only modest (r = 0.27, p < 0.01). The failure to find any double 1-3 labels despite abundant triple labels provides strong evidence against the presence of pauses in bone mineralization between 3 and 34 days.(ABSTRACT TRUNCATED AT 250 WORDS)