The main purpose of the study was to evaluate the use of sperm morphology assessment by strict criteria on the post-Percoll separated spermatozoa used for oocyte insemination in an in-vitro fertilization programme. This study included a consecutive unselected series of 213 oocyte aspirations in 159 women. In 177 aspirations the patient had tubal infertility and in 36 unexplained infertility. Data have been analysed from 197 aspirations where the semen sample used for insemination had a normal sperm concentration (> or = 20 x 10(6)/ml). A total of 1413 oocytes were aspirated, resulting in 863 oocytes which were fertilized and cleaved (cleavage rate 61%). In all, 492 pre-embryos were transferred in 193 cycles, resulting in a pregnancy rate of 42% per transfer. Sperm morphology evaluation using strict criteria showed that Percoll separation significantly increased the percentage of sperm cells with normal morphology from 7.7 to 11.3%. Sperm morphology analysis showed that Percoll separation decreased the number of sperm samples in the 'poor prognosis pattern' group from 31 to 13% and increased the number of sperm samples classified as 'normal' from 16 to 33%. After Percoll separation the poor prognosis pattern group had a cleavage rate of 46%, which was significantly lower than in the good prognosis pattern and the normal groups. However, the poor prognosis pattern group had a significantly higher pregnancy rate than the normal group (P < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)