The authors have estimated the thickness of the endometrium by measuring it ultrasonically between the 5th preovulatory and the 6th post-ovulatory day. This study, which was carried out as part of an in vitro fertilisation programme (IVF) was performed during 162 cycles of which 22 were spontaneous and 140 were stimulated by Clomiphene and HMG. The endometrium is significantly thicker and grows more rapidly when the cycles are stimulated. There is a correlation between the levels of progesterone in the blood and the thickness of the endometrium on the 2nd post-laparoscopy day. On the other hand there was no relationship between the thickness of the endometrium and with the development or otherwise of a pregnancy after replacing embryos.