In a study to determine the proportion of unrecognised cases and the prevalence of treatable complications of autosomal dominant polycystic kidney disease (ADPKD), 46 probands were identified through genetics and renal clinics in Melbourne, Australia. 321 offspring of the probands who were older than 15 years and had not been previously diagnosed as having ADPKD were identified. 68 (21%) had ultrasound evidence of polycystic kidney disease. Of this previously undiagnosed group, 25 (37%) had one or more treatable complications at the time of diagnosis. The complications included 20 cases of hypertension (diastolic blood pressure 95 mm Hg or above), 7 cases of impaired renal function (serum creatinine 0.12 mmol/l or above), and 4 cases of bacterial urinary tract infection. 8 people had several complications. ADPKD has an important treatable component which is not being treated in a substantial proportion of affected individuals, because the disease is not being diagnosed despite the presence of a positive family history.