Previous studies have suggested that the results of coronary angioplasty are poorer when the attempted lesion has a thrombus associated. With the aim of assessing the results of coronary angioplasty in lesions with thrombus, 1,192 consecutive coronary lesions attempted were prospectively analyzed. Of these, 88 (7%) had associated an angiographic intraluminal filling defect consistent with thrombus (group I) and were compared with the remaining 1,104 lesions (93%) without thrombus (group II). Age (56 +/- 12 vs 59 +/- 10 years) was similar in both groups, but patients in group I required more frequently dilatation after a myocardial infarction (acute phase or following thrombolytic therapy) (36% vs 12%, p less than 0.005) but infrequently for stable angina (6% vs 21%, p less than 0.005). Left ventricular ejection fraction (60 +/- 13% vs 63 +/- 12%) and the number of diseased vessels (1.46 +/- 0.7 vs 1.58 +/- 0.8) were similar in groups I and II, respectively, but lesions in group I were less frequently located in the left anterior descending coronary artery (35% vs 53%, p less than 0.025). Furthermore, lesions in group I were more frequently total occlusions (35% vs 4%, p less than 0.001), and were more severe (94 +/- 6% vs 87 +/- 8%, p less than 0.005), eccentric (81% vs 54%, p less than 0.005), irregular (72% vs 32%, p less than 0.005) and more frequently located at bend points (31% vs 17%, p less than 0.05). Primary angiographic success was lower in group I (79% vs 92%, p less than 0.001) and, after dilatation, the incidence of luminal irregularities (34% vs 15%, p less than 0.001) and early reocclusion (10% vs 1%, p less than 0.05) was higher in this group. However, when patients presenting with total occlusions were excluded from both groups primary angiographic success was similar (90% vs 91%) for groups I and II, respectively. We conclude that: 1) Lesions with intracoronary thrombus usually present other unfavourable angiographic characteristics for dilatation. 2) Results of coronary angioplasty in lesions with thrombus are similar to those obtained in other lesions when totally occluded vessels are excluded.