In 21 of 636 patients with normal renal function and without obstruction of the upper urinary tract, urograms showed that contrast medium filled thin cylindrical structures in the renal pyramids with the same course as that of the collecting ducts. The prevalence of cylinders when the low-osmolality contrast medium iohexol was used was 21 of 158 studies (13.3%) and with the high-osmolality contrast medium amidotrizoate, two of 529 studies (0.4%). The cylinders were more distinct and more numerous with iohexol, while with amidotrizoate most cylinders appeared as striations. The cylinders appeared within 4 minutes from the start of contrast medium injection and remained distinct for 35-50 minutes. The diameter of the cylinders varied between 0.05 and 0.30 mm; their course was almost straight and closely followed the shape of the pyramid. The cylinders proved to consist of the highly opacified collecting duct lumina. Their configuration indicated that a single cylinder might be an individual collecting duct, but conclusive evidence was lacking.