We study general linear relations among Poincaré series of different indices with arbitrary but fixed level and weight. We establish several conditions the coefficients involved must satisfy. In particular, we show under some mild assumptions that two Poincaré series of distinct indices cannot be identically equal unless they are both identically zero.(i) for which n, if any, is the Poincaré series P n identically zero? and, (ii) for which pairs (m, n) of distinct positive integers, if any, do we have P m ≡ P n ?We shall see that these two questions are intimately related. Choie, Kohnen, and Ono [2] and Rhoades [13] have previously studied linear relations among Poincaré series, relating the coefficients c i to the principal parts of certain weakly holomorphic modular forms for SL(2, Z). Here we are concerned with determining the algebraic and arithmetic nature of the coefficients rather than giving an explicit description of them.Two of our principal results are the following.Theorem 1.1. Suppose we have P n,k,q ≡ N i=1 c i P ni,k,q , where P n,k,q is not identically zero and (n, q) = 1. If each c i is an algebraic integer, then n (d(n)) 2/(k−1) (n, n 1 n 2 . . . n N ) 2 , where d(n) is the number of divisors of n.