Clinically aggressive neoplastic development appears to be associated with extensive extracellular matrix biosynthesis. Collagen, non-collagenous protein and elastin from 21 specimens of benign hypertrophic prostate (BPH) and 15 samples of cancerous prostate were determined. Collagen and non-collagenous protein concentrations of BPH were similar to those in prostatic carcinoma. The elastin concentration of well or moderately differentiated prostatic carcinoma was greater than that in BPH specimens. These results may provide an explanation as to the early antecedent or possible aetiology of prostatic carcinoma.