A trial of the flavonoid Catergen (Zyma) has been undertaken in 11 adults with osteogenesis imperfecta (Ol). The only significant clinical or metabolic side-effects were severe headaches, which necessitated the withdrawal of 3 patients from the trial. Patient compliance in terms of palatability of Catergen was good, and 3 of the 8 patients who completed the trial experienced subjective improvement. After 6 months' treatment with Catergen, the abnormally narrow collagen fibrils found in the osteoid region in a pretreatment bone biopsy specimen from a middle-aged man with the common type 1 (autosomal dominant) form of Ol showed a significant reversion to normal diameters. Post-treatment specimens from his 2 affected sons, who exhibited the same defect, showed a similar but less marked response.