Aim Male excess infant mortality is well known but unexplained. In 2004, we reported sudden infant death syndrome ( SIDS) and other infant respiratory deaths showed a ~50% male excess in the United States between 1979 and 2002. This study analyses expanded US data from 1968 to 2010 to see whether infant respiratory deaths still show similar ~50% male excess and may be X-linked. Methods The analysis compared infant mortality data from the US Centers for Disease Control and Prevention, 1968-2010, with 11 World Health Organization International Classification of Diseases ( ICD) rubric groups for respiratory deaths by accidents, congenital anomalies, respiratory diseases and causes unknown. Results The 11 ICD groupings presented male excesses of ~50% and combining the 453 953 US cases produced a male fraction of 0.6034, a 52.1% male excess. A further 72 380 non- US respiratory cases showed a similar 0.6055 male fraction, a 53.5% male excess. Conclusion The constant ~50% male excess for quite different causes of respiratory death suggests they all have a common terminal event and that is acute anoxic encephalopathy. We hypothesise that this constant male excess phenomenon must be caused by a single X-linked gene, with a recessive condition, leading to a predisposition to succumb to acute anoxic encephalopathy. [ABSTRACT FROM AUTHOR]