We have previously published evidence that children with St. Jude Medical prostheses on the left side of the heart may not require anticoagulation. Between March 1979 and September 1986, we followed up 48 patients who had no anticoagulant therapy for up to 7 years after valve replacement, an aggregate of 122 patient-years. The 25 male and 23 female patients ranged in age at implantation from 5 months to 21 years (12 +/- 6 years, mean +/- standard deviation). Five patients (all with complex associated malformation) died in the hospital (10%), and nine died late (22%). None of the early and one of the late deaths was associated with a thrombosed prosthesis. During follow-up, seven thrombotic (one mitral, one aortic) or thromboembolic (two mitral, three aortic) events occurred (5.7 +/- 2.1 per 100 patient-years). Of these seven events, five occurred within the last 14 months of the study. There was no relation of these events to age of patient at implantation, age at the time of even, gender, or site of implantation. Concurrently, we have followed up 340 adult patients with St. Jude Medical prostheses who had warfarin sodium (Coumadin) anticoagulation for 875 patient-years. By the end of this study, the children who did not receive anticoagulants were significantly less free of thrombotic and thromboembolic events than the adults who did receive anticoagulants (p less than 0.01).