The purpose of this investigation was to compare the corresponding longitudinal and transverse diameters of tuberculin test and study the influence of reader variations, size of reaction and the age specific infection rates, in order to understand the effect of switching over from the transverse to longitudinal readings. The study was carried out in villages in south India where no BCG or tuberculin testing had been undertaken. A total of 1240 persons were given tuberculin test with 1 TU RT 23 in both longitudinal and horizontal diameters. The indurations were read after 2-4 days by two readers independently. The study showed that though the longitudinal diameters were bigger than the corresponding transverse diameters, these differences did not influence infection rates calculated at 10 mm or more induration level. In National Tuberculosis Institute (NTI), Bangalore the practice of reading transverse diameter was altered to longitudinal diameter in the epidemiological surveys as it was comparatively easier to read the longitudinal diameter. Obtaining almost similar infection rates at the 10 mm or more level of induration in this study, irrespective of readers and diameters has minimized the effect of the changeover from transverse to longitudinal diameter reading in the epidemiological surveys at NTI. This would also not pose any problem in comparing the results of NTI studies with other research studies by any national or international organisation where transverse diameters have been measured.