Glycolic acid peeling is known to improve photoaging processes such as wrinkling and roughness, but this effect has not been clearly defined, even though functional activation of fibroblasts has been suggested. The study was aimed to determine the effects of glycolic acid and malic acid (AHA: alpha hydroxy acid) on cultured dermal fibroblasts. Whether it directly increases cell proliferation may be an important factor influencing the production of extracellular matrix such as type I collagen. Cultured human skin fibroblasts were treated for 24 hours with glycolic acid and malic acid at different concentrations (10(-4), 10(-5), 10(-6) M), and cell proliferation was measured by MTT assay. Then quantitative analysis of collagen synthesis was performed by PICP (Procollagen Type I C-peptide) enzyme immunoassay and radioisotope (3H-proline) labelled collagen assay. The results showed increased cell proliferation and collagen production in response to glycolic acid in a dose dependent manner. The range of cell proliferation and collagen production were significantly higher with glycolic acid treatment than with malic acid or control. It was suggested that the favorable effects of glycolic acid treatment on aging skin were mediated by increased cell proliferation in addition to functional activation of fibroblasts.