Analyses of the aminoterminal propeptide of type III procollagen (PIIINP) in the cerebrospinal fluid (CSF) of 55 children and five young adults without any structural central nervous system (CNS) lesion are reported. The concentration was age-dependent, in that infants and small children had quite high values, whereas the concentration remained relatively constant after the age of 1.5 years. The concentrations of PIIINP in the CSF of 44 children with acute lymphoblastic leukemia (ALL) were prospectively determined at the time of diagnosis and during treatment, since deposition of type III collagen is known to occur during fibroproliferative responses triggered by inflammation. Chemical arachnoiditis is known to be associated with intrathecal methotrexate therapy in children with leukemia. The mean concentration in these children at diagnosis (5.8 micrograms/l +/- SD 2.8 micrograms/l) did not differ from that in age-matched controls (6.7 micrograms/l +/- SD 3.2 micrograms/l). Depending on type of the disease, the children were treated according to two different protocols. PIIINP concentrations were significantly higher during the therapy phases which included intrathecally administered methotrexate (P less than 0.001) than at diagnosis of the disease. Corticosteroid treatments were always associated with a significant decrease in PIIINP concentrations (P less than 0.01 and P less than 0.001 in the two groups, respectively), irrespective of the therapy phase. The results suggest that an increase in PIIINP concentration in the CSF of children with ALL is an indicator of a fibroproliferative response in the arachnoid. Corticosteroids may repress this response and possibly also prevent the development of adhesions in the arachnoid.