Objective: To examine the etiologies of covariation of childhood sleep problems and other behavioral/emotional problems in young children. Method: The parents of more than 6,000 twin pairs provided information on their twins' anxiety, conduct, and hyperactivity at ages 3, 4, and 7 by completing the Strengths and Difficulties Questionnaire. Information on sleep problems was obtained at age 3/4. Results: Phenotypic correlations in preschool years between sleep problems and anxiety, conduct, and hyperactivity ranged from 0.17 to 0.22. These correlations were largely due to shared environmental factors (0.72-0.96) and less to heredity (0.05-0.26) and nonshared environment (0.00-0.06). Sleep problems at age 3/4 years predicted at age 7 years anxiety ([beta] = .12, p < .001), conduct problems ([beta] = .09, p < .001), and hyperactivity ([beta] = .07, p < .001), after accounting for stability of these problems. These predictive associations were also mainly mediated by shared environment (0.16-0.91), although genetic (0.02-0.84) and nonshared environmental (-0.03-0.09) factors were also influential. Conclusions: The modest associations between early sleep problems and later behavioral/emotional problems are largely accounted for by common psychosocial risk factors. Further research on which specific risks may mediate these connections is needed. J. Am. Acad. Child Adolesc. Psychiatry, 2004;43(6):744-751. Key Words: twins, sleep, behavioral/emotional problems.