Partial sleep deprivation (PSD) exerts transient antidepressant efficacy. As a potential mechanism of action an enhancement of dopaminergic neurotransmission within the CNS is discussed. Because genetic variations influencing neurotransmission could have an impact on therapeutic outcome and stability of improvement, we investigated the functional insertion/deletion (I/D) polymorphism of the angiotensin-converting enzyme (ACE) gene to examine a possible influence on the dopaminergic pathway. We included 56 patients with major depression (DSM-IV). Psychiatric ratings including the HAM-D6 scale were assessed prior to and after PSD and related to the different genotypes. The ACE I/D polymorphism was determined following PCR amplification using genomic DNA. A total of 58.1% of the patients were PSD responders. As expected, the therapeutical effect of PSD was transient and most patients experienced an exacerbation of depressive symptoms on day 2. Subdivision according ACE gene variants showed a significantly less pronounced relapse of symptoms in ACE gene D-allele carriers (P=0.02). Our results give first hints that the ACE I/I genotype, probably influencing dopaminergic neurotransmission, could be an indicator for relapse after PSD. This should result in earlier and more intense additional therapeutic interventions in this group of patients.