We studied how subjects with mild cognitive impairment (MCI), early Alzheimer's disease (AD) and age-matched controls learned and maintained the names of unfamiliar objects that were trained with or without semantic support (object definitions). Naming performance, phonological cueing, incidental learning of the definitions and recognition of the objects were tested during follow-up. We found that word learning was significantly impaired in MCI and AD patients, whereas forgetting patterns were similar across groups. Semantic support showed a beneficial effect on object name retrieval in the MCI group 8 weeks after training, suggesting that the MCI patients' preserved semantic memory can compensate for impaired episodic memory. The MCI group performed equally well as the controls in the tasks measuring incidental learning and recognition memory, whereas the AD group showed impairment in this respect. Both the MCI and the AD group benefited less from phonological cueing than the controls. Our findings indicate that word learning is compromised in both MCI and AD, whereas long-term retention of newly learned words is not affected to the same extent. Incidental learning and recognition memory seem to be well preserved in MCI. (Contains 4 tables and 3 figures.)