The synthesis of type I collagen, the major component of the skin, is known to be modulated in aging and in various skin diseases and treatments. In vivo analysis of type I collagen expression, however, is difficult because of the low cell density of the dermis and the small amount of RNA obtainable from skin biopsy specimens. We present here a quantitative polymerase chain reaction method for the quantification of pro alpha 1(I) collagen mRNA in skin punch biopsy specimens. The targeted mRNA and a synthetic RNA as an internal standard were co-amplified together with the same primers. Collagen synthesis was found to decline after birth, so that the amount of pro alpha 1(I) collagen mRNA in the skin of 5- to 58-y-old donors was 17-37% of that in fetal skin. Slot-blot hybridization also indicated that the amount of pro alpha 1(I) collagen mRNA was much lower in adult skin than in fetal skin. In samples from lesional skin of two granuloma annulare patients, the number of pro alpha 1(I) mRNA molecules was increased 4- or 5-fold compared with values from nonlesional skin of the same patients. The method presented is a highly sensitive polymerase chain reaction application, requiring only very small amounts of total RNA.