Background The proposed causal association between complex stenoses and acute coronary syndromes remains speculative. Methods We studied the relationship between coronary stenosis morphology, stenosis progression, and evolution of symptoms in 34 consecutive patients, 21 presenting with chronic stable angina and 13 with acute coronary syndromes. Stenoses (n=103) were classified as concentric (symmetric narrowing), eccentric (asymmetric with smooth borders), and complicated (asymmetric with rough borders and overhanging edges). Progression was assessed by computerized coronary arteriography 15+12 months after initial evaluation. Results The proportion of stenoses that progressed (P<0.05), but a large proportion remained unchanged (67% in patients who developed an acute syndrome and 56% in those who remained stable). Stenoses progressed more frequently in patients who developed an acute coronary syndrome (62%) during follow-up than in patients who presented with stable angina and remained stable (19%) (P<0.05). However, approximately 40% of patients who developed an acute syndrome showed no stenosis progression. Conversely, 19% of patients who remained stable developed coronary occlusion at the site of a preexisting stenosis, without evidence of myocardial infarction. Conclusions Patients who develop acute coronary syndromes frequently show stenosis progression. In a substantial proportion, however, acute syndromes may develop in the absence of stenosis progression. Conversely, in stable angina patients, stenoses may progress to occlusion without myocardial infarction. Complex lesions have a larger propensity for progression than smooth lesions, but a large proportion remain unchanged over years.