We examined and compared 22 human hearts (6 normal hearts, 4 with hypertrophic cardiomyopathy and 12 after acute ischaemia) for their DNA and collagen content and their cell number. A positive correlation between total heart weight and DNA content was demonstrated in all hearts. The relative DNA content decreased and the rate of polyploidy increased in the infarcted and hypertrophied hearts. An aneuploid DNA content was found only in the infarcted hearts. Both hypertrophied and infarcted hearts showed a significantly higher collagen content than did the normal hearts. There was a positive correlation between collagen content and total heart weight in the hypertrophied hearts but not in the infarcted hearts. In the infarcted hearts, the increase in the relative collagen content in the left ventricle was significantly larger than that in the right ventricle. Heart cell number was higher than normal in some hearts with acute ischaemia, which was possibly a sign of compensatory regeneration of heart cells in hearts damaged by ischaemia.