Five surgical specimens of pelvi-ureteric junction from idiopathic hydronephrotic kidneys were obtained according to the Anderson-Hynes technique and were examined under an electron microscope. Fibrosis and marked alterations of several muscle cells were observed throughout all the specimens. Muscle cell alterations were different form cell to cell, namely: a noticeable development of the rough endoplasmic reticulum and of the Golgi apparatus, a great number of glycogen particles, a wide extension of the smooth endoplasmic reticulum and a disorganization of the filament pattern. The aforementioned alterations could be observed both in 'special' and 'typical' muscle cells; moreover, a reduction in the number and extension of cell-to cell junctions was observed in 'special' muscle cells only. It could be guessed that the modifications observed in the muscle cells and the fibrosis are responsible for the altered motile activity of the upper urinary tract and led to the clinical manifestations of idiopathic hydronephrosis. However, the muscle cell malfunction is probably extended more than in the surgically resected specimens, as in the patients studied it was documented by urodynamic data that the urine flow through the ureter was due to be a wide pelvic-ureteric plasty more than an effective peristaltic contraction of the upper urinary tract.