Although necessary for public health, quarantine has been documented to cause post-traumatic stress symptoms, anxiety, and depression. We designed the present longitudinal study to evaluate the psychological impact of quarantine in Italian community-dwelling adult participants. A sample of 304 Italian community-dwelling adult participants (75.7% female; mean age = 35.28 years) was administered self-reported measures of depression, anxiety and acute stress symptoms at the beginning and at the end of the lockdown. Potential predictors of clinically relevant symptoms at the end of the lockdown were assessed. Specifically, data on gender, civil status, education level, occupation, and area of residence, as well as maladaptive personality domains were collected. More than 43% of participants suffered from the early impact of the lockdown; at the end of the lockdown roughly 32% of participants still reported any clinically relevant depression anxiety, and/or acute stress disorder condition. Clinically relevant acute stress reaction at the beginning of lockdown was a particularly important risk factor for experiencing clinically relevant acute stress, depression, and anxiety at the end of the lockdown. Maladaptive personality domains represent non-trivial predictors of participants' self-reports of clinically relevant depression, anxiety, and acute stress conditions at the end of the lockdown. Excess of female participants and the impossibility of evaluating if participants suffered from any internalizing disorder before the COVID-19 quarantine represent major limitations of our study. Our findings suggest assessment of internalizing disorder symptoms during quarantine may be helpful in identifying people who may benefit from early treatment interventions. Copyright © 2020. Published by Elsevier B.V.