The flow obstruction in mitral valve implants was studied in the immediate postoperative period in 19 patients with Björk-Shiley, Hancock, or Lillehi-Kaster prosthetic mitral valves. The effective valve area (Ae) was used as a measure of the flow obstruction. The blood velocity in the implants was estimated with a non-invasive ultrasound Doppler system. The cardiac output was determined with an indwelling thermodilution catheter. The collected data allowed the determination of Ae. Multiple determinations of Ae, at various cardiac outputs and pulse rate were generally performed in each patient during the first 2--3 postoperative days. The investigation demonstrated that the method employed was useful for the study of mitral implant performance. The obtained values of Ae demonstrated that the flow obstruction in presthetic mitral valve implants is frequently considerable.