By means of probability analysis we have compared the diagnostic value of clinical symptoms, m-mode echocardiographic measurements and peripheral arterial flow, assessed by continuous-wave Doppler, in preterm infants with symptomatic patent ductus arteriosus (PDA). Data were obtained in 29 infants with PDA and in 29 controls. The most sensitive clinical finding was a hyperactive precordium. Bounding pulses and a heart murmur were absent in 15% and 20%, respectively of the patients with PDA. M-mode echocardiographic measurements were rather specific for the detection of a PDA but less sensitive. Diastolic backflow in the brachial and femoral arteries was present in the majority of patients with PDA and absent in about 67% of the controls. The values in probability analysis, however, were too low to base a therapy on these findings. The highest sensitivity and specificity (100% each) was found for a disturbed cerebral blood flow with absent or retrograde diastolic perfusion estimated by Doppler sonography.