The purposes of this study were to compare the Bayley Scales of Infant Development (BSID-II) scores at 8 and 18-22 months adjusted age of a cohort of 47 extremely low birth weight infants and to determine whether there was an association between changes in test scores and infant behavior as measured by the Behavioral Rating Scale of the BSID-II at 18-22 months adjusted age. Psychomotor Developmental Index scores did not differ between the two testing points (p = .17), whereas the Mental Developmental Index (MDI) scores dropped significantly (p = .006). Emotional regulation and low household income were both significantly associated with changes in MDI scores (p = .001 and p = .002, respectively). After adjusting for household income, the association between emotional regulation and changes in MDI scores remained significant (p = .02). Results suggest that infant behavioral characteristics, as well as family socioeconomic status, can adversely affect developmental outcome at 18-22 months adjusted age.