The paper presents a study on large-scale automatic extraction of acronyms and associated expansions from Web data and from the user interactions with this data through Web search engines. We investigate three information sources for extracting and ranking acronym-expansion pairs, as provided by a large-scale search engine: the crawled web documents, the search engine logs, and the search results. We evaluate and compare the acronymexpansion pairs generated from these sources on three dimensions: (1) the precision and recall of each source; (2) the overlap and inclusion among the acronym-expansion sets; and (3) the rank-order correlation of the ordered expansion sets. Our results show that all three data sources play an important role in building a comprehensive up-todate collection of acronym-expansion pairs.