An ultrasonographic evaluation of the endometrium was performed in 158 patients undergoing ovarian stimulation for an in-vitro assisted reproduction programme. Endometrial thickness was evaluated in 109 patients undergoing in-vitro fertilization (IVF) for female indications and in 49 patients undergoing intracytoplasmic sperm injection (ICSI) for male indications. The maximal endometrial thickness was measured on the day of human chorionic gonadotrophin (HCG) administration by longitudinal scanning of the uterus on the frozen image using electronic callipers placed at the junction of the endometrium-myometrium interface at the level of the fundus. Cases in which the endometrial thickness was >/=10 mm were included in group A; cases in which the endometrial thickness was <10 mm were assigned to group B. The age of the patients, serum 17-beta oestradiol concentrations on the day of HCG administration, the length of follicular stimulation, the number of follicles, 17-beta oestradiol concentrations per follicle on the day of HCG and the number of embryos transferred were analysed in each case. When comparing endometrial thickness and results in IVF and ICSI patients, an endometrium <10 mm predominated in IVF patients (27.5%) compared with those undergoing ICSI (16.7%) (P = 0.05); conversely an endometrium >=10 mm was more frequent in ICSI than in IVF patients. The incidence of pregnancy was higher in IVF group A patients (32/79; 41%) than in IVF group B patients (5/30; 17%) (P = 0.03), whereas no significant difference was found between ICSI group A (13/42; 31%) and ICSI group B (3/7; 43%) patients. Thus, a higher percentage of IVF patients had thin endometrium when compared with ICSI patients; thin endometrium was a prognostic indicator of pregnancy only in the case of a female indication for infertility (IVF). A thin endometrium in cases of female infertility may reflect a previous or present uterine pathology, whereas in indications of male infertility (i.e. cases using ICSI), in the absence of any associated uterine pathology, the presence of a thin endometrium is not predictive.