Extremely low birth weight (ELBW; <1000 g) infants have been exposed to stressful intrauterine and early postnatal environments. Even greater early adversity has been experienced by ELBW survivors who were also born small for gestational age (SGA; <10th percentile for GA) compared to those born appropriate for GA (AGA). ELBW survivors, particularly those born SGA, face increased risk for internalizing problems compared to normal BW (NBW; ≥2500 g) controls. Internalizing problems are related to allelic variations in the promoter region of the serotonin transporter linked polymorphic region gene (5-HTTLPR). We followed the oldest longitudinal cohort of ELBW survivors to adulthood. Participants provided buccal cells and reported on internalizing problems, using the Young Adult Self-Report when they were in their mid-20s (ELBW/SGA, N = 28; ELBW/AGA, N = 60; NBW, N = 81) and mid-30s (ELBW/SGA, N = 27; ELBW/AGA, N = 58; NBW, N = 76). The findings indicate that ELBW/SGAs carrying the 5-HTTLPR short allele reported increased internalizing problems, particularly depression, during the third and fourth decades of life. This is the first known report on gene–environment interactions predicting psychopathology among ELBW survivors. Our findings elucidate putative neurobiological pathways that underlie risk for psychopathology.