Refractile bodies are one of the main morphological abnormalities that can be observed in the cytoplasm of human oocytes. In the present studies the characteristics of refractile bodies and the relationship between the size of these structures and developmental competence of the affected oocytes and resulting embryos were examined. The refractile bodies were found to have yellow autofluorescence which was consistent with the typical autofluorescence of lipofuscin. Viewed by transmitted electron microscopy, the refractile bodies showed the conventional morphology of lipofuscin inclusions and consisted of a mixture of lipids and dense granule materials. Large refractile bodies (>5 microm) were positively stained by the Schmorl reaction and were considered to contain lipofuscin. These larger lipofuscin inclusions (>5 microm) were associated with significantly reduced fertilization and unfavorable blastocyst development.