The deck of a graph is the multiset consisting of all unlabelled subgraphs obtained from by deleting a vertex in all possible ways (counted according to multiplicity).
Conjecture If two graphs on vertices have the same deck, then they are isomorphic.
See Wikipedia's Reconstruction Conjecture for more on this problem.
Bibliography
*[K] P. J. Kelly, A congruence theorem for trees, Pacific J. Math., 7 (1957), 961–968.
*[U] S. M. Ulam, A collection of mathematical problems, Wiley, New York, 1960.
* indicates original appearance(s) of problem.