To assess the time course and mechanism of early minimal luminal diameter (MLD) loss, serial angiographic observations were performed. Seventy-four patients (with 74 severe narrowings [ > or = 70%]) with acute ischemic syndromes who had an early loss in MLD of > 0.3 mm at 24 hours after percutaneous transluminal coronary angioplasty (PTCA) also underwent 1 hour post-PTCA angiography. In 12 consecutive patients with early loss 1 hour after PTCA, angioscopy was also performed to assess the mechanism of early loss. The percent diameter stenosis for the 74 lesions was 16.8 +/- 8.4% immediately after PTCA, 35.1 +/- 14.2% 1 hour after PTCA (p < 0.002 vs immediately after), and 41.4 +/- 13.2% at 24 hours (p < 0.10 vs 1 hour after). The MLD also showed similar differences: 2.6 +/- 0.3 mm immediately after to 2.0 +/- 0.4 mm 1 hour after(p < 0.002) to 1.8 +/- 0.4 mm 24 hours after PTCA (p < 0.10 vs 1 hour). In 60 patients (81%), the > 0.3 mm loss was detected 1 hour after PTCA. These 60 patients had no further decreases in MLD at 24 hours (1.9 +/- 0.4 vs 1.8 +/- 0.4 mm at 1 and 24 hours, respectively, p = NS). Adequate angioscopic images available in 11 patients showed that red thrombus was present in 1, minor or multiple dissection in 5, and neither thrombus nor dissection in 5 other patients (consistent with early wall recoil). Thus, in narrowings demonstrating early loss in MLD at 24 hours, 81% showed that the early loss occurred within 1 hour after PTCA. Early loss is not related to thrombus but usually to dissection or recoil.