Several studies have demonstrated that the sensitivity of a commercially available PCR test for the detection of Chlamydia trachomatis (Roche Diagnostics) is affected by the cellular quality of the endocervical swab specimens. The cellular adequacies of 1,633 female endocervical swab specimens were assessed and compared with the results of C. trachomatis detection obtained by ligase chain reaction (LCR; Abbott Laboratories). Specimen adequacy studies and LCR were performed with samples from the same swab, after demonstration of the stability of human epithelial cells in LCR transport medium. Prior to heat treatment of the swab specimen, an aliquot was removed and cytocentrifuged onto a slide. Cell spots were stained and examined at ×400 magnification for endocervical (columnar epithelial or metaplastic) cells and erythrocytes. The overall rate of positivity of the LCR was 6.5% (106 of 1,633 specimens) with pooled specimens (pools of 4 specimens each; reduced cutoff). Of the 1,633 specimens examined, 655 (40.1%) were found to contain one or more endocervical cells. The rate of positivity for C. trachomatis was 10.8% (71 of 655 specimens) among specimens containing endocervical cells, whereas it was 3.6% (35 of 978 specimens) among specimens lacking endocervical cells (P < 0.0001). There was no linear trend between the rate of positivity for C. trachomatis and the number of endocervical cells (P = 0.24). The rate of positivity for C. trachomatis was 5.4% (8 of 147 specimens) among specimens containing large numbers of erythrocytes (100 per high-power field), whereas it was 6.6% (98 of 1,486 specimens) among specimens containing less than 100 erythrocytes per high-power field (P = 0.59). These results show that the sensitivity of the Abbott C. trachomatis LCR test is affected by the presence of endocervical cells. Additionally, they indicate that the presence of a single endocervical cell is as good an indicator of specimen adequacy as the presence of many endocervical cells. The presence of a large number of erythrocytes was not associated with an increased rate of sensitivity of the LCR.