(1) The results of 25 cases of renal cysts in pre- and post-natal infants, studied by microdissection, are reviewed here. (2) Cysts are designated as either corpuscular or tubular depending on the site of origin. One case was the single exception at there were both types of cyst present in approximately equal numbers. (3) Small corpuscular cysts are easily recognized by the presence of the glomerular tuft and/or attachment to the proximal convoluted tubule. In larger cysts the origin was established by position and the association with smaller cystic structures of recognizable type. (4) Definite, constantly occurring individual cystic patterns were observed in the tubular varieties, cysts being located at special sites in the nephron. (5) Cysts occurred in both complete and incomplete nephrons. (6) Various conclusions may be stated: (a) Microdissection shows that cysts vary greatly in form, their relation to other tissues (especially the tubules or renal corpuscles in which they arise) and in the stage of development of the individual, (b) The tubules are usually patent and various sizes of cyst and degree of change are found, indicating that not all the changes occurred at one time, (c) All the evidence suggests that in most, if not all, cystic change occurs in a formed nephron and is not due to embryological aberrations of the ‘non-union’ t