Purpose: To determine whether 3 aspects of the word learning process--fast mapping, retention, and extension--are problematic for children with cochlear implants (CIs). Method: The authors compared responses of 24 children with CIs, 24 age-matched hearing children, and 23 vocabulary-matched hearing children to a novel object noun training episode. Comprehension and production were measured immediately following training (fast mapping) as well as 1 day later (retention). Extension was measured in terms of the ability of the participants to identify new (untrained) exemplars. Results: Compared with their hearing age-mates, children with CIs performed marginally more poorly on fast mapping as measured by the comprehension probe and more poorly on retention as measured by comprehension and production probes. The age-mates improved over the retention interval, but the children with CIs did not. Most of the children with CIs performed similarly to their age-mates on extension, but 2 children underextended, and 5 children failed to understand the task. Compared with younger vocabulary-matched peers, children with CIs did not differ at fast mapping, retention, or extension. Conclusions: Children with CIs demonstrated deficits in word learning, with retention being especially problematic. Their learning did not differ from that of younger children with similarly sized vocabularies. (Contains 4 tables and 3 figures.)