The study was conducted in Children Hospital, Bangkok, Thailand based on admitted ARI ccases in anfectious and non inffectious wards. It was conducted on 1st March 1991 to 31 March 1991, to determine the ARI in children aged under five years. The study was a casecontrol study. The sample size were 120 motherscare takers who had their sick children admitted in the hospital. Sixty mothers were selected as a case group with ARI children and sixty mothers as a control group with out ARI children. The mothers both case and control were seiected to interview about ARI occurrence in children under 5 years of age due to risk factors influencing such as incomplete immunization, low birth weight, malnurishment, exposure to passive smoking, indoor air pollution, family history of chronic illness and socioeconomic conditions, so on. The study was demonstrated that there was significant, association between mothers occupation and occurrence of ARI under five years of age. The result revealed the mothers who stayed at home had less risk of ARI to their children than those who work away from house. (63.3%). Knowledge of mothers was found, that mothers of ARI group children (23.3%) had less than 6 scores, than without ARI group childrens mother (28.3%). This study revealed that pneumonia attributed 63.3% cases of children under five years of age. Thus the pneumonia is the principal cause of mortality among children under five years. The study showed there was no significant association between remained risk factors and occurrence of ARI in children aged under five years, such as family history of current smoking incomplete immunization, nutritional status of children, family history of chronic illness, family income and nursery attendance. The result of this study will be an important guideline for the health planner for prevention and control of ARI.