A method was developed to detect Ureaplasma urealyticum in urine by the polymerase chain reaction (PCR). A 457-bp fragment of the urease gene of U. urealyticum was amplified by PCR. Before PCR, components disturbing the amplification had to be reduced. This was possible by diluting the urine 1 in 10 with distilled water and by the extraction of the U. urealyticum DNA. Urine specimens from 41 patients with systemic lupus erythematosus (SLE) and 21 healthy individuals were treated by the dilution method and investigated by PCR for U. urealyticum DNA. The results were compared with those obtained by culture and the detection rates of PCR and culture were found to be identical. Also there was no difference in the detection rates of U. urealyticum from urine of SLE patients and healthy individuals; 10 (24.4%) of the 41 urine specimens from SLE patients and five (23.8%) of the 21 urine specimens from healthy individuals gave positive results for U. urealyticum. The results of this study do not indicate a decisive role for U. urealyticum in SLE.