In this article the authors present their analysis of the online search patterns within user searching sessions of the National Library of Medicine ELHILL system and examine the user search patterns on the CATLINE database. In addition to the CATLINE analysis, a comparison is made using data previously analyzed on the MEDLINE database for the same time period, thus offering an opportunity to compare the performance parameters of different databases within the same information system. Data collection covers eight weeks and includes 441,282 transactions and over 11,067 user sessions, which accounted for 1680 hours of system usage. The descriptive analysis contained in this report can assists system design activities, while the predictive power of the transaction log analysis methodology may assists the development of real-time aids.