The present study investigates implicit memory and arithmetic reasoning in a group of severely amnesic patients. Priming effects have been observed in the retrieval of arithmetic facts in amnesic patients (Delazer, Ewen and Benke, 1997), but no studies have so far investigated the effect of previous experience in a pure reasoning task. Findings indicate that answering to a first numerical series (e.g, 1 3 5 7, answer "9") facilitates answering to a second numerical series (2 4 6 8, answer "10") including different numbers but sharing the same underlying algorithm (+ 2). Within the model of Langdon and Warrington (1997) priming effects are attributed to the stage of abstract formula generation.