For its I2th symposium the Society for the Study of Inborn Errors of Metabolism met in the ancient and illustrious university town of Heidelberg. The principal topic for discussion was 'Inborn Errors of Calcium and Bone Metabolism' -an apt choice in view of the important advances made in recent years in our understanding of bone disease. The pathways leading to the synthesis of I,2S-dihydroxycholecalciferol, the metabolically active form of vitamin D, have been elucidated, and sensitive and specific methods developed for the assay of parathyroid hormone. At the symposium, distinguished investigators from Europe and North America presented 18 papers which provide an up-to-date review of this important field, and besides contain much practical information not readily available elsewhere. In the third Milner Lecture Professor Charles Dent critically reviewed the rickets group of diseases from the unique perspective of the cases seen in his clinics over the past three decades. A session of free communications, outside the main topic (nine papers) reflected on-going research by members of the Society, particularly in the field of aminoacidopathies, and included contributions on two recently discovered inborn errors of metabolism, oc-aminoadipic aciduria and oc-ketoadipic aciduria.