The Authors report follow-up data (mean 60, range 13-101 months) from 4 patients under two years of age (mean 11.5 months) operated on for prosthetic mitral valve implantation. Two patients were male and two female. Pre-operative diagnosis was partial atrioventricular septal defect with parachute mitral valve in one case and isolated mitral valve anomaly in the others (1 parachute, 2 hammock valves). One bioprosthesis (Liotta n. 23) and three mechanical prostheses (Björk-Shiley) were implanted. Complications occurred in two patients: one case of endocarditis on the bioprosthesis, leading to substitution with a Björk-Shiley valve; one case of acute thrombosis 8 months after implantation in the only pt receiving aspirin as prophylactic therapy. This complication was resolved by intravenous thrombolytic agents. All the patients are alive, in I NYHA functional class and receiving oral anti-coagulants. Instrumental follow-up with eco-Doppler demonstrated normal functioning prostheses in all the patients. A sub-aortic gradient was found in the patient with partial atrioventricular septal defect. Prosthesis-patient mismatch was identified in a patient operated on when she was 6 months old who was followed-up for 5 years.