Previous studies have noted the abundance of collagen in human erectile tissues and the association of altered collagen content with erectile dysfunction. We investigated these notions by studying the collagen characteristics of biopsies from the corpus cavernosum of men who required surgical correction of their sexual dysfunction. Histologic analysis revealed abundant collagen within the erectile tissues. With the exception of patients with Peyronie's disease and priapism, only mild alterations in collagen architecture were noted in the remainder of the patients. Biochemical quantitation confirmed the histologic study. The mean collagen content represented 47% of total protein in most patients. The proportion rose to 68% and 73% in the patients with Peyronie's disease and priapism, respectively. No statistical difference in collagen content was noted in all the patients studied. Immunohistochemistry revealed collagen types I and IV to predominate in the corpus cavernosum, with type III making up the minority. There were no qualitative changes in collagen ratios with age and disease. We conclude that though collagen is a major component of the penis, there are no changes in its histologic characteristics that can be correlated to senescence or to the etiology of erectile dysfunction.