It has been well established that trabecular bone strength has a power relationship to apparent bone density. It has been argued that the apparent bone density is not the sole factor determining cancellous bone strength, and that bone strength may also depend on the structure of the trabecular network. Various different measures of bone microstructure have been suggested: marrow space star volume; node-strut analysis; connectivity density; trabecular number; and trabecular separation. The aim of this study was to assess which of the histomorphometric measures, trabecular bone volume, marrow space star volume, node-strut analysis, connectivity density, trabecular number, and trabecular separation, give the best prediction of the bone strength. Moreover, a comparison of the suggested "structural" measures was carried out to establish whether they actually quantified the bone structure or something different. Finally, the influence of age and gender was investigated. The material comprised unilateral transiliac crest bone biopsies from 32 females (21-96 years, mean 59.1 years) and 32 males (24-94 years, mean 59.0 years). The bone biopsies were embedded in Technovit 9100, sectioned, and stained with aniline blue. Trabecular bone volume, marrow space star volume, node-terminus ratio, connectivity density, trabecular number, and trabecular separation were measured. A neighboring biopsy was compressed to obtain the compressive strength of the trabecular network. Trabecular bone volume was the measure that gave the strongest correlation with bone strength (r = 0.76 for females and r = 0.86 for males). High correlation was found between trabecular bone volume, marrow space star volume, node-terminus ratio, and trabecular separation--indicating that they carried the same information. A poor correlation was seen between these measures and connectivity density. Connectivity density was the only measure showing gender-dependent behavior. A multiple regression analysis showed that the trabecular bone volume is the best histomorphometric predictor of bone strength, and neither marrow space star volume, node-terminus ratio, connectivity density, trabecular number, nor trabecular separation could improve the predictive value of trabecular bone volume.