Achondroplasia is a specific entity and is no longer considered to be synonymous with short-limb dwarfism. Achondroplastic dwarfs are detected at birth by their short limbs, normal size trunk, and craniofacial abnormalities. Roentgenograms are diagnostic and show caudal narrowing of the lumbar spinal canal, short vertebral pedicles, a narrow sacrosciatic notch, and rhizomelic micromelia. Frequently encountered orthopaedic problems include bowlegs, supinated feet, hip-flexion contractures, fibromyositis, and bursitis. Spinal malalignment, excluding the invariably exaggerated lumbar lordosis, may be expected in 70 per cent of patients. Kyphosis is usually mild, and located in the thoracolumbar area; scoliosis, usually also mild, is predominantly in the lumbar vertebrae. Some of the serious neurological difficulties, encountered in at least 12 per cent of patients, may result from spinal-cord or nerve-root encroachment not seen in normal individuals. This encroachment may be the result of a narrowed spinal canal on the normal sized cord, herniated nucleus pulposus (on the basis of exaggerated lumbar lordosis), angulation of the spinal cord, mushroom spurring of the articular processes, and a recessed or unstable vertebral body. When present, neurological signs are associated with kyphosis, scoliosis, or both, in 72 per cent of achondroplastic dwarfs and do not seem to be dependent upon the age of the individual. In contrast to existing opinions, coxa vara, tibial torsion, and dislocated patellae are not features of achondroplasia and their presence should lead one to suspect some other form of dwarfism. Arthritis of major weight-bearing joints does not appear to be a problem in achondroplasia; at least there is no evidence of an increased incidence of degenerative change compared with normal individuals. In general, angulation osteotomies of the tibiae are to be preferred to bowleg braces when shoe fitting, appearance, or pain due to established poor joint mechanics is a problem. Of the many forms of dwarfism which must be differentiated from achondroplasia, one should include the more severe pseudoachondroplastic spondyloepiphyseal dysplasia and the less severe hypochondroplasia. Thanatophoric dwarfism, rather than achondroplasia, is probably the diagnosis in the severely dwarfed new-born infants dying in the perinatal period.