Different aspects of the histology of luminal epithelial cells were examined quantitatively at the light microscopical level. For the first time, using a variety of morphometric indices, an attempt was made to quantify and compare the cellular events in a group of well defined fertile controls and infertile subjects during the peri-implantation period. The most striking observation was that the majority of the features examined, including all nuclear parameters and most cell dimensions, were similar and did not differ significantly between the fertile controls and the infertile subjects over the period of study. The coefficients of variation of most of the parameters investigated were similar and within a narrow range in both the fertile and infertile population, indicating little variation either between individuals at a particular stage or between subjects on different days. This low inter-subject variability may suggest that the cellular events which we have been able to quantify at the light microscope level in the uterine surface epithelium around the anticipated time of implantation are precisely regulated. Such precise regulation does not mean that changes do not occur in luminal epithelial cells during this time, but that those which do occur are well controlled and co-ordinated. Possible changes in biochemistry, physiology and ultrastructure of these cells remain to be determined.