Urokinase-type plasminogen activator (u-PA) is one major activator of plasminogen. It is also involved in tissue remodelling, angiogenesis, cell migration, and tumour metastasis. In adults, increased u-PA levels have been identified in patients with chronic liver disease. No data exist for u-PA plasma levels in children. In the present study, u-PA plasma levels were measured by ELISA in 95 healthy children and adolescents aged 7 months to 17 years. We found a median value of 1.06 ng/ml u-PA (range 0.43-15.78 ng/ml), which is similar to that found in adults (2-20 ng/ml). No differences between males and females were recorded. In addition, we determined u-PA plasma levels of 16 patients with severe or mild type I plasminogen deficiency and found a median value of 1.06 ng/ml (range 0.69-7.7 ng/ml). Normal plasma u-PA values in healthy children and adults are virtually no different from those reported in adults.