There are only a few studies on the adverse effects of inhaled corticosteroids on the skin in asthmatic patients. Therefore, we evaluated the effect of inhaled corticosteroids on de novo collagen synthesis of skin and bone, skin thickness and the total amount of skin collagen. Twenty seven consecutive new asthmatic patients, on a moderate dose of budesonide or beclomethasone dipropionate, were invited to take part in this prospective study. Radioimmunological analyses of aminoterminal propeptides of type I and III procollagens (PINP, PIIINP, respectively) in suction blister fluid (SBF) of skin and in serum and carboxyterminal propeptide of type I procollagen (PICP) and cross-linked carbox terminal telopeptide of type I collagen (ICTP) in serum were performed at entry and after 3 and 6 months of inhaled corticosteroid treatment. Ultrasound measurements of skin thickness at two sites were performed at entry, at 3 and 6 months and after 1-2 yrs of inhaled corticosteroid treatment in 20 patients, six of whom had been prescribed one or more courses of oral corticosteroids. Skin hydroxyproline of punch biopsies was determined to measure the total amount of skin collagen (males, at entry and at 6 months). Skin thickness and the total amount of skin collagen on the abdomen were unchanged after 1-2 yrs of inhaled corticosteroid use. A slight decrease was observed in the upper arm skin thickness, especially in those subjects who had received inhaled plus oral corticosteroids. The procollagen propeptide concentrations (PINP, PIIINP) were markedly decreased in SBF at 3 months and remained at this level at 6 months. In serum, a slight decrease was seen in the PINP, PIIINP and ICTP concentrations at 3 and 6 months. In conclusion, inhaled corticosteroids decrease the collagen synthesis of skin and bone, but skin thickness and the total amount of collagen in skin are not changed markedly after 1-2 yrs of treatment.