This study describes and exemplifies generally applicable design-based stereological methods for obtaining quantitative estimates of the numbers and sizes of capillaries, cardiomyocytes, and cardiomyocyte nuclei in immersion-fixed human left ventricles (N = 6). The design-based stereological methods are valid in all cardiac investigations onto quantifying changes in structure and function as seen under various conditions such as during development, aging, hypertrophy, and following ischemia/reperfusion. The applied principles of unbiased stereology were as follows: 1) uniform random sampling was taken at all levels, also in respect to orientations, for estimates of length and mean sizes. 2) All global structural quantities were estimated as total quantity = density x volume of the left ventricle. As an example, the left ventricle contains 1.5 x 10(9) capillaries with a total length of just below 200 km. 3) Stereological methods were used for estimating the volume density, surface area density, and length density of capillaries and cardiomyocytes. The numerical density of cardiomyocyte nuclei and capillaries was estimated, using the optical and physical disector, respectively. 4) In all local quantities, "size" was estimated either directly, using unbiased estimators to obtain the average individual size and size distribution parameters, or indirectly, using the relationship that: average size = total quantity/total number. In the six hearts constituting this study, we observed the anticipated correlation between left ventricular volume and global estimates such as total number of capillaries. There were no correlation between local quantities and total left ventricular volume (e.g., average star volume of individual cardiomyocytes).