The internal mammary arteries (IMA) are considered to be the superior conduit in coronary bypass grafting (CABG). Anomalies of an IMA can influence the surgical technique and results; their true incidence is not well known. The IMA's were visualised angiographically in 262 consecutive patients undergoing cardiac catheterisation prior to CABG. Satisfactory visualisation was possible of 459/524 IMA's studied (88%). A total of 118/459 (26%) surgically significant anomalies was observed in 79/262 patients (30%): common origin of another large artery in 48/459 (11%), large side branches in 41/459 (9%), tortuosity in 21/459 (5%), atypical course or origin in 5/459 (1%), atherosclerotic lesions in 2/459 (0.4%) and spasticity of an IMA in 1/459 (0.2%). Angiographic visualisation of the IMA's resulted in modification of surgical strategy in 11/262 patients (4%); meticulous preparation because of difficult or atypical IMA anatomy was necessary in 68/262 patients (26%). These results demonstrate that significant anomalies of the IMA which might--when unrecognized--jeopardize IMA-flow after CABG are relatively common; they might escape detection during IMA take-down but can be diagnosed by angiography during catheterisation.