ABSTRACT Research has shown that adults are skilled at learning new words and meanings. We examined whether learning new meanings for familiar words affects the processing of their existing meanings. Adults learnt fictitious meanings for previously unambiguous words over four consecutive days. We tested comprehension of existing meanings using a semantic relatedness decision task in which the probe word was related to the existing but not the new meaning. Following the training, responses were slower to the trained, but not to the untrained, words, indicating competition between newly-acquired and well-established meanings. This effect was smaller for meanings that were semantically related to existing meanings than for the unrelated counterparts, demonstrating that meaning relatedness modulates the degree of competition. Overall, the findings confirm that new meanings can be integrated into the mental lexicon after just a few days’ exposure, and provide support for current models of ambiguity processing.