Background: Much of the evidence suggesting that loneliness is a risk factor for socio-emotional adjustment difficulties comes from studies with older children and adolescents. Comparatively less is known about the mental health implications of loneliness in early childhood. The goals of the present study were to provide additional convergent validity of the "Loneliness and Social Dissatisfaction Questionnaire for Young Children" (Cassidy & Asher, 1992) and explore potential gender differences in relations between loneliness and behavior problems in early childhood. Methods: A multiple source assessment of 139 kindergarten-aged children (M[subscript age] = 64.76 mos., SD = 4.48) was undertaken, including parental ratings, behavioral observations, child interviews, and teacher ratings. Results: Overall, loneliness was positively associated with anxiety, aggression, and peer exclusion. However, several gender differences were also observed. Conclusions: Loneliness may be a marker variable for both early internalizing and externalizing problems. However, gender differences in the behavioral associates of loneliness are also evident.