Prolyl hydroxylase (PH) and procollagen III peptide (PIIIP) levels in 43 serum samples from various collagen diseases including 19 progressive systemic sclerosis (PSS) cases were simultaneously assayed and compared to those of 22 age-matched psoriasis patients and 20 healthy controls in order to examine whether they can serve as useful parameters of fibrotic changes, especially in the skin. Serum PH levels were determined with enzyme immunoassay and serum PIIIP levels with radioimmunoassay. The average values of PH were 109% (n = 3, (p less than 0.001; generalized morphea, GM), 104% (n = 1; overlap syndrome, OS), 57% (n = 6; mixed connective tissue disease, MCTD), 48% (p less than 0.05; PSS), 33% (n = 5; dermatomyositis, DM), 28% (psoriasis), and 9% (n = 9; systemic lupus erythematosus, SLE), higher than the control levels, while those of PIIIP were 82% (OS), 57% (MCTD), 42% (p less than 0.01; PSS), 11% (GM), and 5% (DM) higher and 22% (psoriasis) and 28% (SLE), lower than the controls. These results indicate that both values were higher in the diseases with skin fibrosis and lower in those without it, suggesting that both assays generally reflect the state of actual progression of fibrosis at the point of determination. These two values had considerable correlation; however, GM, psoriasis and SLE patients had relatively higher values of PH than of PIIIP. This result suggests that the PH assay may also reflect the state of connective tissue activation; therefore the PIIIP assay may be more specific for pathological fibrotic process.(ABSTRACT TRUNCATED AT 250 WORDS)