Prevalence of low birth weight infants in Japan has been increasing, although infant mortality is the lowest in the world. We conducted a population based case-control study to clarify risk factors for low birth weight infants in Japan. Information was obtained by questionnaire. The response rate was 78.9% from 402 cases and 804 controls. The majority of low birth weight infants belongs to full-term case group. This is considered to be an important factor for low infant mortality in Japan. We examined potential risk factors by a multiple logistic regression model. Lower maternal academic career and toxemia were identified as significant risk factors for pre-term low birth weight infants, while maternal shorter height, lower prepregnancy weight, maternal smoking and employment during pregnancy and hypertension were identified as significant risk factors for full-term low birth weight infants. The Japanese small physique and prevalent hypertension are considered to be causes for the high prevalence of low birth weight infants. J Epidemiol, 1994
4: 91-98. © 1994, Japan Epidemiological Association. All rights reserved.