A socioeconomic status (SES) and social support questionnaire was administered to the families of 559 singleton infants in a National Institute of Health collaborative study conducted at five university medical centers. A subset of six items measuring both SES and social support (which accounted for 87 percent of the common variance) was developed and entitled the Environmental Quality Index (EQI). These scores were summarized and separated into quartiles, and the performance of infants in the quartiles was compared on 9-, 18-, and 36-month outcome measures. Significant differences were found between EQI levels on 8 of 10 outcome measures, indicating the clinical usefulness of this new instrument. The EQI is appended. (Author/RH)