At CRYPTO 2008 Stam [7] made the following conjecture: if an m+s-bit to s-bit compression function F makes r calls to a primitive f of n-bit input, then a collision for F can be obtained (with high probability) using r2(nr−m)/(r+1) queries to f. For example, a 2n-bit to n-bit compression function making two calls to a random function of n-bit input cannot have collision security exceeding 2n/3. We prove this conjecture up to a constant multiplicative factor and under the condition m′ :=(2m−n(r−1))/(r+1)≥log2(17). This covers nearly all cases r=1 of the conjecture and the aforementioned example of a 2n-bit to n-bit compression function making two calls to a primitive of n-bit input.