This paper evaluates the retrieval effectiveness of query expansion strategies on a MEDLINE test collection using Cornell University's Smart retrieval system. Two expansion strategies are tested on their ability to identify appropriate MeSH terms for user queries: expansion using an inter--field statistical thesaurus and expansion via pure retrieval feedback. These expansion strategies do not require prior relevance decisions. The study compares retrieval effectiveness using the original unexpanded and the alternative expanded user queries on a collection of 75 queries and 2,334 MEDLINE citations. Retrieval effectiveness is assessed using eleven point average precision scores (11-AvgP). Expansion by retrieval feedback gives the best improvement of 16.4% over a baseline performance of 0.5169 11-AvgP. Query expansion using the inter--field thesaurus gives a significant but lower performance improvement (9.9%) over the same baseline. This study recommends query expansion using retrieval feedback for adding MeSH search terms to a user's initial query.