Of 900 biopsies made with a small suction curet 457 from 329 women were analyzed to show the progression of the endometrium from the high-estrogen early-proliferative phase, immediately following menstruation, through the late proliferation occurring about Day 12 foll owing menstruation. Classifications from Days 14 to 27 are much easier to make because the 1st sign of progestin effect causes significant change. On Day 15 vacuolization of the cytoplasm and beginning migration of the nuclei toward the surface of the cells is seen, leaving a lucid zone underlying the nuclei. On Day 17 this zone is well-marked and the nuclei above it are lining up to lie later each beside the other near the middle of the cell. On Day 19 the row of nuclei has sunk distally near to the base of all the cells. On Day 18 a beginning edema of the stroma is seen, first in patches; by Day 21 it is generalized and the last vestige of the zona pellucida has disappeared. On Day 21 the process of secretion, which began about Day 17, is well advanced. From Day 21 on the signs of hormonal action are seen not primarily in the gla nds but in the stroma and vascular system. Edema has become generalized by Day 21; on Day 23 the stroma cells are larger, more thick-walled, and more numerous; by Day 25 the edema is replaced by masses of contiguous large cells with large pale nuclei and much cytoplasm; and on Day 26 size of the stroma cells has extended so that almost all the interglandu lar cells are contiguous. On Day 27 this solidification of the most sup erficial stroma becomes complete, the glands are widely dilated, the epithelium approaches the cuboidal, the vascular system is highly developed, and the arterioles and venules are engorged with blood. Late on Day 27, just before menstruation, lymphocytes and polymorphonuclear leukocytes appear in great numbers and erythrocytes appear in clumps. T he whole predeciduum is infiltrated, the stroma nuclei become pale, neutrophils appear in large numbers, the tissue disintegrates, and menstruation occurs. Understanding of this normal process can aid in clinical diagnosis of menstrual disorders. A series of biopsies taken with a small surgical curet does little tissue damage and can reveal hormonal imbalance, thin stroma associated with menopause or hypoplastic endometrium, and such pathological conditions as tuberculous endometritis and cancer of the endometrium. In the 3 cases in which a woman with unsuspected early pregnancy was biopsied, no harm was done. This study showed amenorrhea is usually due to deficient follicular development but may be present even if a proliferative endometrium shows a high degree of follicle activity; in the absence of pregnancy there is never a persistent corpus aluteum.