Electron microscopy of human cyclical endometrium indicates that there is considerable loss of gland epithelial cells with formation of apoptotic bodies in the late secretory, premenstrual and menstrual phases, but also to a lesser extent in the proliferative phase. Apoptotic bodies containing nuclear fragments may be recognised on light microscopy as basophilic or Feulgen positive granules. These granules accumulate in greatest numbers towards the end of the cycle and may be used as an additional aid in dating curettings.