Background: Retinaldehyde has been shown to exert antibacterial activity in vitro. Aim: This study evaluates the effect of retinaldehyde on Propionibacterium acnes both in vivo and in vitro. Methods: Microbial minimal inhibitory concentrations (MICs) of retinaldehyde and retinoic acid were determined on reference strains of P. acnes. In vivo activity of daily topical application of 0.05% retinaldehyde on the P. acnes density was evaluated after application in a single-blind randomised study. Results: MICs of retinaldehyde were 4 mg/l for P. acnes No. CIP179 and CIP53119 and 8 mg/l for P. acnes No. CIP53117. In contrast, the MICs of retinoic acid were superior to 128 mg/l for these three strains. In vivo, retinaldehyde-treated areas displayed a significant decrease in counts of viable P. acnes as compared with the untreated areas with a median decrease of 102 log P. acnes/cm2 after 2 weeks of daily application. Vehicle alone had no effect. Conclusion: The MIC of retinaldehyde against P. acnes suggests a direct antibacterial activity. Daily topical application of 0.05% retinaldehyde is associated with a clear reduction of the P. acnes density.