A quantitative analysis of trabecular bone structure is presented, based on omnidirectional distributions of paths across (a) trabeculation and (b) marrow cavities. The omnidirectional distributions, which take into account structural anisotropy, are generated from measured distributions of paths. Representative examples are given, together with values of two commonly quoted structural parameters, the ratio of endosteal surface to bone volume and percentage bone volume. Data on the biological and age variations in the third lumbar vertebra are also presented and an index of trabecular anisotropy suggested. Finally, the results are compared with those of other workers and estimates of useful skeletal parameters given.