BACKGROUND: Growing numbers of post-adolescent females are suffering from treatment-resistant or relapsing adult acne forms, therefore requiring the definition of safe and effective treatment options for this burdening disease.OBJECTIVES: To assess the efficacy of azelaic acid 15% gel (AzA) vs. no treatment during maintenance therapy of female adult acne and to compare its efficacy and safety vs. adapalene 0.1% gel (AD) during a 9-month period (3-month treatment and 6-month maintenance treatment).METHODS: A total of 55 women between 18 and 45 years with adult acne were included in this investigator-blind trial and randomized into three groups receiving AzA gel b.i.d. for 9 months (AzA9M, n = 17) or AzA gel b.i.d. for 3 months followed by a 6-month observational phase (AzA3M, n = 19) or AD gel once daily for 9 months (AD9M, n = 19). Parameters of efficacy, safety and patient-related factors were analysed.RESULTS: The reduction in lesion counts, severity and Dermatology Life Quality Index score was significant (P < 0.05) and comparable between groups during the treatment phase, while dryness and scaling were significantly lower (P < 0.05) in group AzA9M vs. AD9M. During maintenance, AzA9M was superior to AzA3M in the control of inflammatory lesions (P = 0.008) and total lesions (P = 0.014) at week 24. From week 12 to week 36, a mild relative increase in inflammatory lesions could be observed in all groups. In AzA3M, this increase exceeded that of AzA9M by 23.1% (P = 0.109), while the difference of total lesions diverged to 30.8% (P = 0.038). No significant differences could be detected between AzA9M and AD9M. Group AzA9M was non-inferior to AD9M (non-inferiority margin of 50% for the confidence limit for the relative effect) in the control of inflammatory acne lesions.CONCLUSIONS: AzA15% gel is a safe and effective treatment and maintenance treatment of female adult acne with non-inferior efficacy to AD 0.1% gel in the control of inflammatory acne.