BackgroundInternalizing problems are common in young children, often persist into adulthood, and increase the likelihood for subsequent psychiatric disorders. Problematic attachment, parental mental health problems, and stress are risk factors for the development of internalizing problems. COVID-19 lockdown measures have resulted in additional parental burden and especially their impact on preschool children has rarely been investigated as of now. The current study examined the impact of sustained preschool attendance, parental stress, and parental mental health on internalizing and externalizing problems during COVID-19 lockdown measures in a sample of preschool children in Germany. Methods and findingsN = 128 parents of preschool children filled out a one-time online survey about children's internalizing problems, externalizing problems, and attachment for three time points: before a nation-wide lockdown (T1), during the most difficult time of the lockdown (T2) and after the lockdown (T3). Additionally, parents answered questions about their own depressive and anxious symptomatology for the three time points and parental stress for T1 and T2. Linear-mixed effect models were computed to predict children's internalizing / externalizing behavior. Preschool children showed a significant increase in internalizing and externalizing problems over time, highest at T2 with small decreases at T3. Parental depressive and anxious symptomatology increased significantly from T1 to T2, but also remained high at T3. Parental stress levels were comparable to community samples at T1, but attained average values reported for at-risk families at T2. Linear-mixed effect models identified higher parental stress, parental anxiety, attachment problems, parental education, and less preschool attendance as significant predictors for internalizing and externalizing problems in preschoolers with more specific associations shown in separate models. A limitation is the retrospective assessment for the times T1 and T2. ConclusionsPreschool children's mental health is strongly and negatively influenced by the ongoing COVID-19 pandemic and its lockdown measures. Sustained preschool attendance may serve as a protective factor.