Asplenia syndrome is characterized by complex congenital heart defects, asplenia and abdominal heterotaxy. Recent interest in the syndrome has been increased by new knowledge arising from animal models and by continuing improvements in surgical outcome in childhood. To further elucidate the embryologic timing and mechanisms of the asplenia syndrome, 32 necropsy cases were reviewed and 487 published autopsy cases were reanalyzed at the hospital. The most common congenital heart defects were atrial septal defects, common atrioventricular canals and conotruncal anomalies. With use of current information on the timing of normal development, it was hypothesized that most defects originate at Streeter Horizon XIII; patients averaged 3.2 Horizon XIII defects, more than at any other stage. Distribution was unimodal. Extracardiac anomalies also exhibited a developmental spectrum. Because the normal spleen develops by Horizon XIII, asplenia, the sine qua non of the syndrome, originates then or earlier. Abnormal pulmonary lobation occurred in 80% of cases, with right isomerism occurring most often; pulmonary branching asymmetry also originates at or before Horizon XIII. Abdominal heterotaxy occurred in 72% of cases, but the timing of origin is unclear. Anomalies of other systems, including genitourinary, musculoskeletal, endocrine, and nervous systems, develop later (typically XV to XXIII); specific anomalies were less frequent, although much more prevalent than in the general population. It is concluded that asplenia syndrome is a focal developmental disturbance in laterality which occurs primarily at Horizon XIII.