Two groups of patients, each consisting of 40, were investigated for the occurrence of mycoplasmas in the urethra, bladder and upper urinary tract. Mycoplasmas were isolated significantly more frequently from the bladder urine of patients with chronic pyelonephritis than from patients with non‐infectious urinary tract diseases. Furthermore, mycoplasmas were isolated from the upper urinary tract of 5 patients with chronic pyelonephritis, while mycoplasmas could not be cultivated from the upper urinary tract of patients with non‐infectious urinary tract die