Bone marrow biopsy (BMB) is generally performed either on the anterior superior iliac spine (ASIS) or the posterior superior iliac spine (PSIS), the choice between these two sites depending largely upon practice at individual centres. No previous study has attempted to ascertain which of these two sites is preferable for needle BMB. We studied 72 biopsies, of which 36 were of the PSIS and 36 of the ASIS, measuring the length of the cylinder and the area of the histologic section. We asked those patients who had undergone BMB at both sites which had been the less painful. The cylinders obtained from the PSIS were found to have a significantly greater length and area than those obtained from the ASIS (P < 0.00001). Of the 13 patients who underwent BMB at both sites, 11 reported the PSIS biopsy to have been distinctly less painful (P = 0.012). We conclude that needle BMB of the PSIS provides samples of greater length and area, and is less painful, than that of the ASIS.