The coronary slow flow phenomenon is an angiographic finding characterized by delayed distal vessel opacification in the absence of significant epicardial coronary disease. Case-control and observational studies of patients with this phenomenon were conducted to determine the associated clinical features and prognosis. Patients with coronary slow flow (n = 47) differed from controls (n = 47), more often being current smokers (32 vs. 9%; p < 0.01) and presenting with rest pain requiring urgent admission (74 vs. 21%; p < 0.001), usually to the coronary care unit (66 vs. 17%; p < 0.01). During a median 21-month follow-up of 64 slow flow patients, 84% had recurrent chest pain. Based upon these findings and those of previous investigators, it is speculated that coronary slow flow is a new disease entity characterized by acute but recurrent perturbations of microvascular function.