Early attachment has both protective and risk factors as regards later mental health. In Finland, insecure-avoidant-type attachment is overrepresented. Does this indicate a risk for emotional and behavioural problems at preschool age? In this study, we examined and compared the association between attachment and mental health in Finnish singletons and twins. The attachment of 27 singletons and 58 twins with both parents had previously been examined at the age of 18 months using Preschool Assessment of Attachment criteria. At the age of 4, the children's emotional and behavioural symptoms were assessed using Child Behavior Checklist questionnaires reported by parents. The mothers and fathers reported significantly more behavioural and emotional problems at preschool age in singletons than in twins. Insecure-avoidant-type attachment to the mother was significantly associated with higher scores concerning withdrawal, somatic problems and total internalising symptoms among singleton preschoolers, whereas among twins, no such correlations were observed. The results thus suggest that twinship is a protective factor in the trajectory from toddler avoidant attachment to internalising problems at preschool age.