Obstetric departments which provide service for a large number of patients from different parts of the country and socioeconomic backgrounds like the Obstetric Department of Chulalongkorn hospital, need to develop rapid laboratory tests which can cope with the volume of work and yet provide sound laboratory data for management decisions. We, therefore, undertook a study of the suitable Rapid Plasma Reagin (RPR) test as a diagnostic tool for syphilis in 9,347 pregnant women who attended antenatal clinics at this institute from August 1984 to May 1985. The RPR test was used in addition to the routine serological tests for syphilis namely the VDRL, TPHA and/or FTA-ABS. Analysis of results confirmed that the RPR test fulfilled all laboratory results for clinical requirements. The results from RPR and VDRL were not significantly different. Their sensitivities were 90.86 and 89.95 per cent, their specificities were 99.57 and 99.68 per cent, their positive predictive values were 82.11 and 85.92 per cent, their negative predictive values were 99.8 and 99.78 per cent, and their accuracy was 99.39 and 99.47 per cent respectively. In our study it was found that the RPR test could provide a laboratory diagnosis in 60-90 minutes; in the same morning period when 15-20 prenatal patients were seen for the first time. The above results suggest that the RPR test is a rapid and reliable tool which is particularly suitable for syphilis screening in a busy antenatal clinic. The test enabled all 197 patients with syphilis in pregnancy to be treated promptly and without any loss of follow-up.(ABSTRACT TRUNCATED AT 250 WORDS)