Human skin fibroblasts were subjected to treatment with a Neodymium:YAG laser at 1060 nm with varying levels of energy determined by a reproducible method of dosimetry. DNA synthesis in the cells was measured by the incorporation of [3H]thymidine, and collagen production was monitored by the synthesis of nondialyzable [3H]hydmxyprohne after incubation of cells with [3H]proline. Using energy levels equal to 1.7 × 103 l/cm2, a significant reduction in DNA synthesis was noted, while the cells remained viable as tested by the trypan blue exclusion lest. With energy levels higher or equal to 2.3 × 10 l/cm2, the suppression of DNA synthesis was accompanied by cell nonviability. The collagen production, when measured immediately following the treatment with 1.7 × 103 l/cm2, was markedly reduced, and similar effects were observed with higher energy levels. However, when the cells were tested for collagen production at 20 hours following laser treatment, there was a significant decrease in collagen production at energy levels as low as 1.1 × 103 l/cm2, a dose that did not affect DNA synthesis or cell viability. Thus, the results indicate that the Nd:YAG laser can selectively suppress collagen production without affecting cell proliferation. These observations suggest that laser treatment could potentially be used to reduce collagen deposition in conditions such as keloids and hyperlrophic scars.