Lysinuric protein intolerance (LPI) is an autosomal recessive disease caused by defective transport of the cationic amino acids lysine, arginine, and ornithine at the cell membrane. About 80 patients with LPI have been described worldwide, almost half of them in Finland. The symptoms appear in early childhood as a failure to thrive, growth retardation, muscular hypotonia, and episodes of stupor after protein-rich meals. Twenty-nine Finnish patients (current median age 24.8 years, range 3.7-47.9 years) over a mean follow-up time of 18.1 years (range 1.2-27.2 years) had 57 fractures after minor trauma, mostly in childhood. Their 440 skeletal radiographs showed severe osteoporosis (13/29), controversially abnormal thickening of cortex of the metacarpals (7/29), or thin cortices of the long bones (5/29), endplate impression of vertebrae (8/29), rickets-like metaphyses (2/29), or early destruction of cartilage (3/29). Skeletal maturation was delayed by 1-5 years in 23 of 24 patients. There was no correlation between fracture incidence, radiological bone structure, and delayed skeletal maturation.