Serum aminoterminal propeptide of type III procollagen (PIIINP) was studied in 73 psoriatics receiving methotrexate and in 11 selected for trial with methotrexate or etretinate. 72 of the patients on methotrexate were also investigated with liver biopsies. The highest PIIINP value was found in a patient with ascites and her PIIINP decreased after medication was discontinued. Psoriatics with fibrosis or cirrhosis in their liver biopsies had a significantly higher mean PIIINP than patients without fibrosis, who had the same mean value as psoriatics prior to treatment. Based upon the individual data together with data from serial PIIINP investigations of 11 patients studied during treatment, it is concluded that PIIINP can be utilized as a valuable non-invasive test for liver fibrogenesis in methotrexate-treated psoriatics. PIIINP is not specific for the liver, but the study indicates that the number of liver biopsies can be reduced in psoriatics on methotrexate who have normal levels of PIIINP.