Innominate grooves are delicate, inconstant speculations on the colonic margin which may be seen on barium enemas; their origin and significance are matters for discussion. Three hundred and fifty-three double contrast enemas (divided into four groups, depending on the techniques uses) were studied in respect of the presence, localisation and circumstances under which innominate grooves could be seen. The results showed that the incidence of grooves depended on the technique used and varied between 91% and 3%. The most common localisations were the descending and sigmoid colon and in the transverse colon, near the hepatic flexure. The grooves could be seen only towards the end of the examination. It follows that innominate grooves are a normal phenomenon in a healthy mucosa, which can be recognised under optimal circumstances. It is probably due to visualisation of the relief pattern of the mucosa, the delicate folds being particularly noticeable in relation to lymph follicles. Innominate grooves should not be misinterpreted as abnormal findings, but must be distinguished from somewhat similar changes in the colon due to inflammatory disease.