This prospective study included 698 children with diverse socioeconomic and ethnic characteristics, and controlled for 8 demographic and environmental factors, including SES, ethnicity, educational stimulation in the home (HOME), mother's intelligence, and mother's smoking. Subjects were recruited at birth, and monitored for ear status every 2-4 wk in the home; 379 subjects were assessed for cognition with the Stanford Binet 4th Edition (SB4) at 3 yr and 294 subjects at 5 yr. Duration and temporal patterns of bilateral OME in the first 3 yr of life were related to cognitive status, using the SAS General Linear Models procedure. A direct relation was found between duration of bilateral OME (proportion of days of OME from 0-36 mo) and SB4 Composite and Nonverbal Reasoning/Visualization Factor scores at age 3, but not at age 5. We estimate that a reduction in OME duration by 50% during the first 3 yr would be associated with a 3.5 point difference in SB4 Composite scores. Statistical clustering analysis of our sample revealed four groups, based on the temporal pattern of their bilateral OME experience from 0-36 mo: Low OME, Early OME(peaking at 0-6 mo), Later OME (peaking at 6-12 mo), and High OME. GLM analyses revealed no direct effects, but several moderated effects, of OME cluster on cognitive development at 3 yr, with none at 5 yr. SB4 Composite scores were negatively related to OME for the Later OME group. This relation was greater for Euro-American and Hispanic children than for Afro-American children. Children in the Later OME group who were of low SES had lower SB4 Composite scores. In general, high SES appeared to blunt the potentially harmful effects of OME. HOME was more positively related to intelligence in the Low and Early OME groups, suggesting that a stimulating home environment is more advantageous if a child has little OME, or little after 6 mo of age. For none of the cognitive measures did children in the Low or Early OME groups appear to be more adversely affected by OME. These results suggest that prolonged OME from 6-12 mo puts children at risk for cognitive delays at 3 yr which may resolve by 5 yr. A study of the same sample at 7 yr of age is in progress.