Topical corticosteroids are used extensively to treat inflammatory skin diseases. Long-term use, however, may be associated with adverse effects such as skin atrophy. New steroids have been developed with the objective of increasing efficacy and reducing the incidence of adverse effects. Mometasone furoate (MMF) is one of these new derivatives. The aim of our study was to compare the effects of MMF and betamethasone-17-valerate (BM-17-valerate) on collagen synthesis in human skin in vivo. Fifteen healthy male volunteers applied MMF, BM-17-valerate and vehicle for 1 week to different areas of abdominal skin. Suction blisters were raised on these areas, and a control site, and procollagen propeptide (PICP, PINP, PIIINP) levels in the suction blister fluid were measured by radioimmunoassay. Skin thickness was measured ultrasonically by Dermascan A at the end of the treatment period. The levels of the three propeptides in suction blister fluid were reduced to similar extent by MMF and BM-17-valerate. The 1-week treatment period had no detectable influence on skin thickness. We conclude that MMF and BM-17-valerate decrease collagen synthesis to the same extent in human skin in vivo.