After bone marrow transplantation in children, it is essential to detect secondary liver diseases and hepatotoxic effects of immunosuppressive therapy. These can be revealed by cytolytic syndromes sometimes associated with cholestasis. It is therefore important to find an early and specific cholestasis enzymatic marker. A retrospective study of the changes in levels of biological parameters has been carried out in 13 children who underwent one or more bone marrow transplantations. During the 3 months following bone marrow transplantation, all patients developed liver injury characterized by an early and very elevated 5'-nucleotidase activity (sometimes more than 40 times the upper reference limit), a moderate increase in alkaline phosphatase activity, a variable increase in alanine aminotransferase activity and inconstant changes in total bilirubin levels. These results show that cytolytic syndrome and cholestasis are often associated with increases in 5'-nucleotidase and alkaline phosphatase activities. These increases are not correlated, probably due to the influence of therapy on the synthesis and release of both enzymes in the liver. 5'-nucleotidase seems to be the best marker for the detection and follow-up of liver disease in this patient group.