Osteonectin, also termed BM40 or SPARC (secreted protein, acidic and rich in cysteine) is a multifunctional glycoprotein involved in tissue mineralization, cell-extracellular matrix interactions as well as angiogenesis. It has been suggested that osteonectin may play a key role in the process of tumoral invasion and metastasis in certain malignancies. In this study, we reviewed the clinical records and the histopathologic slides of 188 thin cutaneous malignant melanomas (< or = 0.75 mm). Among them, 12 cases underwent progression and were selected for the study. Osteonectin expression was investigated by immunohistochemistry in these 12 patients and 24 matched controls who did not undergo progression. Osteonectin staining was correlated with clinical outcome and other clinicopathologic parameters. Progression-free and disease-specific survival rates were calculated with the Kaplan-Meier method and their differences were evaluated by the log rank test. Overall, immunoreactivity for osteonectin was found in 23 (63.8%) cases. Eighteen cases (50%) displayed staining in 1% to 50% of neoplastic cells whereas five cases (13.8%) showed a diffuse positivity in more than 50% of the tumor cells. Osteonectin expression was significantly correlated with risk of progression (P = .01), incidence of distant metastases (P = .005) and survival (P = .03). There was a higher incidence of osteonectin-positive tumors in cases that did experience regional lymph node metastases versus those cases that did not, but that difference did not reach statistical significance (P = .06). No significant correlation was found between osteonectin expression and other clinicopathologic features, including age, sex, site, histotype, Clark's level, presence of regression, presence of inflammatory response, and tumor growth phase. Our data showed that osteonectin expression is a predictor of clinical outcome in thin cutaneous melanomas.