Colonization with Pseudmonas cepacia in patients with cystic fibrosis (CF) has been associated with increased morbidity and early death, compared with colonization by P. aeruginosa. The mode of acquisition of P. cepacia is not fully understood, although person-to-person spread appears likely. Recent epidemiologic studies support the importance of social contact in the spread of P. cepacia among patients with CF. This study was undertaken to investigate the epidemiology of P. cepacia colonization among patients with CF attending the CF clinic at our center. Isolates of P. cepacia were collected from patients at two CF treatment centers, including ours. Additional isolates were collected from patients without CF in the hospital ICU, from other teaching hospitals, and from the environment. Profiles of enzymes were obtained by ultrathin polyacrylamide gel electrophoresis of P. cepacia extracts. A predominant electromorphic type (ET) was found among P. cepacia isolates from patients at both centers, suggesting a common source or person-to-person transmission. The majority of hospital isolates fell into a single, different ET. Surveillance swabs of respiratory equipment at our CF clinic did not grow P. cepacia. Attendance of patients at CF summer camp correlated strongly with P. cepacia colonization (P < 0.0001).