Morphometric analysis of human ovarian decidual cells was performed with a Videoplan computer, and mean values were established for the area and perimeter of cellular processes and organelles. Two-hundred forty electron micrographs representing 160 cells were analyzed. The mean decidual cell area was 218.7 μm2 of which 34.5 μm2 was occupied by the nucleus (15.8% of the cytoplasmic area); the nucleus contained 1.74 μm2 of nucleolar material (0.8%). The endoplasmic reticulum occupied 13.63 μm2 (6.2%). Mitochondria occupied 7.3 μm2 (3.3%) and the Golgi network 5.49 μm2 (2.5%). Decidual secretory bodies occupied 0.91 μm2 (0.42%) and cytoplasmic processes 1.89 μm2 (0.94%). The remainder of the cytoplasm, containing inclusions and cytoskeleton, represented 71% of the cell area. Perimeter measurements indicated an average decidual cell was surrounded by 87.8 μm of plasma membrane. The mean nuclear membrane measured 28.3 μm (representing 32.3% of the plasma membrane, pm, or 4.1% of total cellular membranes, cm). Outer mitochondrial membranes measured 156.6 μm (178% pm, 23.5% cm); endoplasmic reticulum membranes measured 350.3 μm (400% pm, 52.6% cm); Golgi membrane measured 30.77 μm (35% pm; 4.5% cm) and membrane surrounding secretory bodies measured 9.8 μm (11.2% pm; 1.4% cm). A mean of 280 secretory bodies per ovarian decidual cell was calculated. The plasma membranes of evaginated cytoplasmic processes represented 22.3% of the total pm (19.6 μm or 2.9% cm). A mean of seven such processes was observed per 87.8 μm of plasma membrane (160/cell). These morphometric data provide a baseline for comparisons of human ovarian decidual cells with uterine decidua, in vivo and in vitro, as well as with decidual cells of other species.