Introduction
 During the course of primary health care activities in Khammouane province of Lao PDR by non- profit organization, international support and partnership for health, an exceedingly high infant mortality rate was recognized in some villages. As thiamine deficiency was strongly suspected for the main cause of the high mortality, the household survey with an emphasis on nutritional aspect and the measurement of thiamine level in the blood of mothers as well as their breast-milk were carried out for the confirmation.
Methods
 The survey was conducted at Sibounhouane sub-district (Group 1) with the highest infant mortality and Hatkhamhieng sub-district (Group 2) with the lowest mortality. Fifty families of the two groups, each consisting of randomly selected 25 families with less than one year old infant(s) were interviewed by using prepared questionnaire. The body weight of both mother and infant was measured and blood and milk were taken from the mother.
Results
 The ethnic minorities of the group 1 consisted of Lao Lum (44%) and Lao Theung (56%), whereas only Lao Lum in the group 2. The economical indicators and the mother's educational level in the group 1 were inferior to those in the group 2. There was no significant difference in the methods of preparing staple glutinous rice, likely to flux thiamine and intake of thiaminases between the two groups. High infant mortality rate was documented in the group 1 whose clinical symptoms were mostly consistent with those of infantile beriberi. The concentration of thiamine in the blood and milk of mothers in both groups were considerably lower and significant reduction of thiamine level was observed in the group 1 than the group 2. Nevertheless, both groups of those infants tended to be inadequately fed with the foods except for breast-milk during their early infantile phase.
Conclusions
 The low concentration of thiamine in the blood and milk of mothers was strongly suggestive that infantile beriberi due to thiamine deficiency was the principal cause of the infant mortality in the study region. We hereby surmise that thiamine deficiency could be extensively prevalent in Lao PDR than the currently studied areas because of their traditional preparing methods of staple dietary rice, postpartum food taboos and intake of thiaminases. For the prevention of infant death due to severe thiamine deficiency, necessary measures including the exchange of information among the institutions concerned, adequate thiamine supplementation and nutritional counseling have to be urgently adopted.