To determine whether a change in the size of a scintigraphic perfusion defect is of prognostic significance in dilated cardiomyopathy (DCM), we performed thallium-201 myocardial scintigraphy twice for each of 13 patients with mean intervals of 45 +/- 18 months. Seven patients showed clinical deterioration (Group D) during the follow-up period of 58 +/- 22 months; whereas, 6 patients improved clinically as well as echocardiographically (LVDd < or = 5.5 cm and %FS > or = 20%) (Group I). 1. The initial scintigraphy showed that 3 patients had large defects (LD), 2 had multiple small defects (MSD) and 2 had no defects (NL) in Group D. Among 6 patients in Group I, one had LD, 3 had MSD and 2 had NL. 2. The second scintigraphy showed that in 5 patients in Group D, the defects were progressively enlarged and in the remaining 2 who had no defects at the initial scintigraphy, new defects developed (MSD). Among 6 patients in Group I, no patient had enlargement of defects. In 4 patients with defects (1LD, 3MSD), the sizes in their defects reduced. 3. At the initial study, all 7 patients in Group D fitted NYHA functional class III-IV; while, in Group I, 4 of the 6 patients fitted class III-IV and 2 patients class II. In the second study, all 7 patients in Group D remained in class III-IV; whereas, all 6 patients in Group I fitted class I-II. Five of the 7 patients in Group D died of heart failure after the second study.(ABSTRACT TRUNCATED AT 250 WORDS)