Estimates of the association of chromosome aberrations with various adverse health outcomes are as follows: embryonic and fetal death (5–28 weeks), 33%; stillbirths, 5–10%; infant and early childhood death, 5–7%; retardation (IQ < 50), 20–35%; (IQ 50–69), 5–15%; criminality and retardation or psychiatric disturbance in males, 3%; other criminality, 0.3–0.8%; birth defects, 2.5–8%; heart defects, 10%; subfertility or infertility, 2–8%; multiple miscarriages, < 1 to 13%; male pseudohermaphroditism, < 25%; and primary amenorrhea, 25%. Variations in rates reflect statistical fluctuations and variations in the selection of populations for study. The estimates are based on currently used techniques; cryptic structural rearrangements may also contribute significantly to these outcomes.