Although the average fertilization rate in most in-vitro fertilization (IVF) centres is approximately 60-70%, there are cases of complete or virtually complete fertilization failures. The aim of our work was to study the fertilization and the subsequent cleavage characteristics of 1-day-old human oocytes treated by intracytoplasmic single sperm injection (ICSI) after failing to fertilize during the standard IVF procedure. A total of 115 metaphase II 1-day-old unfertilized oocytes were collected from 23 patients. No additional treatment was applied to the oocytes or to the semen sample. A single spermatozoon from the patient's husband was injected into the cytoplasm of each of these oocytes 21-33 h after ovum retrieval. Injected oocytes were observed at 16-18 h and again 42-44 h after the ICSI procedure. Of the injected oocytes, 92% (n = 106) were intact after ICSI, 38% (n = 44) had two distinct pronuclei and there was no difference in the fertilization rate of oocytes when andrological and non-andrological patients were compared. Similarly, there was no difference in the fertilization rate after ICSI where patients with acceptable or good (> 15%) fertilization after standard IVF were compared to patients who had poor (< or = 15%) fertilization after IVF. There was no significant difference in the sperm concentration or in the progressive forward motility (a + b motility) in these groups except where a + b motility of andrological and non-andrological patients was compared. The majority (84%) of the normally fertilized oocytes cleaved and most (77%) of these embryos showed < 20% fragmentation 2 days after the ICSI procedure.(ABSTRACT TRUNCATED AT 250 WORDS)