One hundred patients (32 male) aged 5 months to 82 years (median 32 years) underwent 106 surgical procedures for 112 mechanical prosthetic valves obstructed by a thrombus (n = 61) or pannus (n = 7), or both (n = 44), between January 1, 1980 and December 31, 1989. The position of the obstructed prosthesis was aortic in 51 patients (48%), mitral in 49 (46%) and both aortic and mitral in 6 (6%). The types of obstructed prosthetic valves were Björk-Shiley (n = 51), St. Jude (n = 41) and Medtronic-Hall (n = 20). The time interval between valve replacement and obstruction ranged from 6 weeks to 13 years (median 4 years). Of 63% of patients in whom coagulation variables were available at the time of obstruction, 70% were receiving inadequate anticoagulant therapy. In 63% of the procedures the patient was in New York Heart Association functional class IV. Two patients underwent preoperative thrombolysis with incomplete results. Operative procedures included valve replacement (n = 81), valve declotting and excision of pannus (n = 23) and aortic valve replacement and mitral valve declotting (n = 2). The early mortality rate was 12.3% (13 patients), and there was no difference between surgery for mitral prostheses (12.2%) versus aortic prostheses (13.7%). The perioperative mortality rate was 17.5% (11 of 63 patients) in patients in functional class IV and 4.7% (2 of 43 patients) in those in functional classes I to III (p less than 0.05). For valve replacement, the mortality rate was 12% (10 of 81 patients) and for declotting of the prosthesis 13% (3 of 23 patients).(ABSTRACT TRUNCATED AT 250 WORDS)