We prove that cutting-plane proofs which use split cuts have exponential length in the worst case. Split cuts, defined by Cook, Kannan, Schrijver (1993), are known to be equivalent to a number of other classes of cuts, namely mixed-integer rounding (MIR) cuts, Gomory mixed-integer cuts, and disjunctive cuts. Our result thus implies the exponential worst-case complexity of cutting-plane proofs which use the above cuts.