A computer system which can be used to perform three dimensional reconstruction of dispersed cancer growth from serial sections is described. The system was used to reconstruct and analyze melanoma nodules growing in the left upper lobes of the lungs of seven mice following intravenous inoculation of a single cell suspension of melanoma cells. The analysis shows a great variability in the tumor size within a single mouse, and also in different mice. The preponderance of tumors (40 out of 50) were subpleural and extended to the surface of the lung at some point. An analysis of the spatial distribution of the tumors was performed which indicates that the tumors may not be randomly dispersed in the lungs. The mechanisms which could lead to the observed results are discussed.