Descriptive statistics, Chi square analysis, and multivariate logistic models were used to examine the factors related to infant mortality in Shanghai city proper and the suburbs in 1990. Mortality samples included 309 cases of infant mortality and 309 controls (same sex, same hospital, and within 5 days of the same birthday) from the city and 223 cases and 223 controls from the suburbs. Data was obtained from the annual mortality reports of the Shanghai Sanitation and Antiepidemic Station. 60.71% (323) of infant deaths were males, of whom 62.46% (193) were from the city proper and 58.30% (130) were from the suburbs. In the city, 61.50% were neonatal deaths (under 28 days) and 45.7% of all infant mortality were early neonatal deaths (under 7 days). 74.2% of total neonatal deaths were early. The respective percentages in the suburbs were 67.2% for neonates and 76.6% for early neonates. The percentage of neonatal and early neonatal deaths out of total infant mortality was higher in the suburbs. The neonatal mortality in the city reflected a decline from preceding years. 18.8% of infant mortality was due to pneumonia, and 17.55% was due to suffocation. Suburban mortality was most often from pneumonia (16.6%) and suffocation (15.2%), followed by premature birth, congenital malformation, and congenital heart disease. Premature births were higher in the suburbs. Rankings in the city were similar to the suburbs: pneumonia, suffocation, neonatal diseases, premature birth. 82.5% infant deaths from the city and 75.7% of infant deaths from the suburbs had received treatment as in or out patients. About 20% received no treatment. Higher mortality was found to related to low birth weight, twin pregnancy, premature birth, medication during pregnancy, few prenatal visits, placenta previa, and infant postnatal diseases. Unrelated factors included parents' age, and maternal height, weight, and addiction to smoking or alcohol. Logistic analysis showed significant factors for city deaths were birth weight, gestational cycle, birth defect, and paternal education. Suburban explanatory factors were birth weight, gestation cycle, maternal occupation, medical system, and maternal prenatal visits. Colinearity was found in the suburban model between maternal occupation in farming, self-financed medical care, and low frequency of prenatal visits.