Legionella pneumophila continues to play a role in both community- and nosocomially-acquired pneumonia. We investigated the ability of L pneumophila to adhere to various types of materials such as those found in the hospital air-cooling and portable water distribution systems. Through the use of a unique sampling apparatus, we were able to regularly acquire planktonic and sessile samples and determine the numbers of bacteria present in both populations, in vitro and in situ. Portions of these apparatuses could be aseptically removed for examination by scanning electron microscopy, or for the determination of the number of viable adherent L pneumophila. The number of bacteria present in each sample was determined by direct plate count, with presumptive L pneumophila colonies being positively identified by direct fluorescent antibody staining techniques. The results demonstrated that not only are legionellae capable of colonizing various metallic and nonmetallic surfaces but that they are preferentially found on surfaces. Surface-adherent bacteria may play a profound role as a reservoir of these potential pathogens in aquatic environments. Furthermore, these results suggest that any comprehensive legionella monitoring program must include not only water samples but also an examination of the adherent populations.