The 1984 OSHA Standard for ethylene oxide (EtO) mandates medical surveillance under various circumstances. When performed, medical surveillance for EtO must include a complete blood count (CBC) with differential leukocyte count. This requirement is based on reports of EtO-associated absolute lymphocytosis and other hematologic effects. This paper describes our experiences in providing EtO medical surveillance for a 300 bed hospital over a 6 year period. We observed an apparent relative lymphocytosis which persisted over 3-4 years in sterilization workers with documented TWA personal EtO exposures averaging 0.07 ppm. In addition, three workers had a history of acutely toxic overexposure to EtO as a result of a sterilizer malfunction. These workers became symptomatic following the high accidental overexposure, but did not show absolute lymphocytosis or altered patterns in the relative lymphocytosis. Finally, a cross-sectional comparison of the CBC data from the EtO-exposed workers to data from non-EtO-exposed hospital workers showed no significant differences, ruling out an association of the relative lymphocytosis with EtO exposure. These observations led us to review the basis for the inclusion of the CBC in routine EtO medical surveillance. Our experience, review of the literature on EtO-associated lymphocytosis and anemia, and review of the literature on the use of the CBC with differential as a screening test suggest that the leukocyte differential may not be useful in routine medical surveillance for EtO exposure.