In this study we report the changes of interstitial collagen in the human non-infarcted interventricular septum after a myocardial infarction as well as in hypertrophic human hearts with or without hypertension. The collagen amount was determined with the Sirius Red morphometry technique, which enabled us to perform these studies on routinely processed, paraffin embedded sections. The collagen amount was significantly increased in the septum of infarct patients as compared to non-infarcted controls (P < 0.001). The collagen amount in the septum of the hypertensive hypertrophy group (HH) was significantly increased as compared to the non-hypertensive hypertrophy group (NHH) (P < 0.01). The collagen content in the NHH was not significantly different from the controls in the infarct group, while the collagen amount in the HH showed no significant difference to the collagen content in the infarct group. The results indicate that collagen deposition is increased in the non-infarcted myocardium after a myocardial infarction, as well as in the hypertensive hypertrophied myocardium. The data suggest that the appearance of excessive collagen is not mediated by cardiac hypertrophy per se, but that the underlying cause, infarction or hypertension, is the significant factor.