BACKGROUND: Boxers are predisposed to subaortic and pulmonic stenosis (SAS, PS). Screening of puppies may be useful in estimating the risk of their developing a defect that potentially compromises life expectancy or exercise tolerance. HYPOTHESIS: Presence of SAS or PS in adult Boxers can be predicted by auscultation and Doppler echocardiography at 9-10 weeks of age. Animals: Eighty-five Boxer puppies examined at 9-10 weeks of age and at 12 months of age. Methods: Prospective, longitudinal observational study. Auscultation by stethoscope and continuous wave-Doppler echocardiography for peak velocities (V(max)) in the aorta (Ao) and pulmonary artery (PA). Results: Intensity of heart murmurs in puppies correlated with V(max)Ao and V(max)PA in adults. V(max)Ao and V(max)PA in puppies correlated with V(max)Ao and V(max)PA in adults, respectively. From puppy to adult, V(max)Ao increased and V(max)PA remained unchanged. The negative predictive value for absent or only a soft (< or =II/VI) murmur in puppies being associated with V(max)Ao and PA < or = 2.4 m/s as an adult was 90% and < or =3.5 m/s 100%. The negative predictive value of a V(max)< or = 2.4 m/s as a puppy still being < or =2.4 m/s as an adult was 94% for Ao and 96% for PA, and of a V(max)< or = 3.5 m/s, 99% for Ao and 100% for PA. Conclusions and Clinical Importance: Even though V(max)Ao increases during growth in Boxer puppies, indicating relative narrowing of the aorta, puppies with V(max)Ao < or = 2.4 m/s do not usually progress to clinically have relevant SAS at 12 months of age.