An algorithm has been developed to determine the surface of an organ in vivo from the 3-D discrete intensity measurements of radionuclide uptake provided by Single Photon Emission Computed Tomographic (S.P.E.C.T.) data. The surface, which need not be convex, is covered by triangular tiles whose vertices are determined to lie on the organ boundary from examination of a density function along lines normal to the organ surface. This approximation is repeatedly refined by replacing the longest edge, and the two tiles that share it, with two new edges sharing a new vertex near the midpoint of the replaced edge, and four new tiles. Volume is defined as the sum of the products of the area of each tile and the dot product of its outward unit normal and centroi d vector. The tile and vertex data also lend themselves to a computationally straightforward projective display of the tiled surface, viewed and illuminated from arbitrary angles. A cine loop of these projections at equally spaced viewing angles provides an informative representation of organ structure and function.