Using contrast-enhanced phototrichogram (CE-PTG) at monthly intervals during 48 months, we measured the duration of the hair cycle, i.e. anagen, catagen and telogen at the exclusion of exogen. Exogen, a recently identified phase of the hair cycle, is characterized by weakening of anchorage of the club hair to the surrounding epithelium. The processing of the club hair terminates at the time of exogen hair release, i.e. hair shedding. We combined a noninvasive exogen sampling before each CE-PTG so that the area contained only anagen, catagen and telogen hair or empty follicular openings. During the first 24 months of this study, natural regression of hair cycling in early i.e. preclinical stages of androgenetic alopecia (AGA) in androgen sensitive areas was documented. Shortening of the hair cycle of thicker hair characterized progression of AGA. During the next 24 months, finasteride (1 mg/day) was introduced into the system. Shortening of the hair cycle was reversed by finasteride in androgen sensitive sites as long as the affected follicle was able to produce a thick hair fiber at the time of treatment initiation. Compared to the baseline period, responding follicles did not produce thicker hair. On average, they initiated active growth more rapidly by reducing the duration of the lag phase by 40%. The duration of the anagen phase of thick hair showed an average 23% increase. In this particular experiment, the already miniaturized follicles producing thinner hair (<40 microm thickness) at the time of finasteride introduction regressed further on treatment. Our results seem to indicate that reversal of 'hair loss' by finasteride probably means that the terminal type follicles that are functionally deficient--a stage of reversible hypotrophy--will be reactivated by two non-mutually exclusive mechanisms: faster regrowth followed by extension of the duration of anagen. In our study, there was no clear evidence in favour of reversal of miniaturized hair into terminal hair. This new interpretation indicates that miniaturized hair follicles may be an important diagnostic marker of AGA in males but also that it might be less contributive to the therapeutic response to finasteride. Our results highlight that precise measurement of terminal type hair follicle functionalities opens up avenues for the selection of 'drug-responsive organs' in the human scalp in vivo and these may possibly serve to predict 'quality of response to treatment'.