In a way, all acne can be considered hormonal. Acne vulgaris is a dynamic process that involves the hair follicle and sebaceous gland (pilosebaceous unit). Broadly speaking, there are three main factors that can cause acne: a hyperactive sebaceous gland, hyperkeratinization, and bacterial influence. Except for perhaps bacterial colonization, the other factors can be influenced by hormones. In teenagers, a lot of the acne seen is due to an imbalance between sebum overproduction (generated by testosterone) and the capacity for the maturing follicle to handle all this excess oil. Once the skin normalizes or matures, and the sebum production is stabilized, the skin clears up.
Adult acne is almost always simply 'hormonal.' In women, ovulation goes hand-in-hand with a surge in hormones, particularly luteinizing hormone (LH). In one study during the menstrual cycle, the sebum output level was significantly higher and the pore size was larger in the ovulation phase than in other parts of the cycle. (Roh,M. et al. BJD 2006;155(5):890-894). So ovulation increases oil production as well as inflammation, which can stimulate or worsen a breakout.