The serum concentration of laminin P1 antigen was determined in 32 patients with various myeloproliferative disorders, including 19 patients with idiopathic myelofibrosis. The serum concentration of the aminoterminal propeptide of type III procollagen (PIIINP) was measured concomitantly in 27 patients. Serial laminin measurements were carried out in 25 patients. The median serum laminin concentration in patients with acute disease, i.e. acute myelofibrosis and patients in a transforming disease phase was significantly higher (1.58 U/ml; range 1.15-2.07) as compared with patients with chronic disease (1.02 U/ml; range 0.75-1.76; p= 0.012) and healthy control subjects (1.13 U/ml; range 0.75-1.67; p = 0.00015). In individual patients serum laminin covariated closely with serum PIIINP, the leucocyte count and LDH. The median serum laminin concentration in patients with a huge spleen was significantly lower than in the patient group with a normal spleen size/previous splenectomy. Pronounced splenomegaly was particularly prevalent in patients with chronic disease implicating splenic enlargement as a significant extrahepatic site of laminin uptake/degradation in myeloproliferative disorders.