The study released Tuesday found at least half of all deaths of babies under a year old on remote outer islands were not reported to the Ministry of Health last year.
It said the infant death rate was 90 deaths per 1,000 births or three times the national level of 30. A rate of 90 is among the worst in the world and in line with some of the poorer African nations.
The study by the Ministry of Health, the Economic Policy, Planning and Statistics Office and the New Caledonia-based Secretariat of the Pacific Community (SPC) reviews population data for the 2004 to 2006 period.
"Quite scary to think that of 51 infant deaths in 2006, 25 (or half) were not recorded in your registration system in the outer islands," SPC statistics official Gerald Haberkorn said.
He made the comment in a memo to Health Secretary Justina Langidrik and planning director Carl Hacker.
Infections, premature births and respiratory problems are common causes of deaths among babies in the Marshall Islands.
The report also suggested the population of the Marshall Islands is likely to be 7,400 lower than the official estimate of 60,000, which had not taken account of emigration to the US.
An average of more than 1,000 Marshallese have left the country each year since 1996, but this was never previously included in population estimates, according to the report.
The implications of the new report include a lowering of the life expectancy of Marshallese and a rise in the rates of some diseases such as tuberculosis and leprosy.
While a 2006 Ministry of Health reports set life expectancy at 70.6 years for women and 67 for men, the new report recommends lowering these to 67.4 for women and 63.7 for men.