Most health care workers (HCWs) are aware of the rationale for hand hygiene procedures, yet failure to adhere to guidelines is common. Little is known about factors that motivate HCWs to practice hand hygiene. The purposes of this study were to (1) estimate adherence to hand hygiene recommendations; (2) describe relationships among motivational factors, adherence, and intensity of nursing unit activity; and (3) test an explanatory model for adherence to hand hygiene guidelines based on the theory of planned behavior (TPB). A longitudinal, observational design was used to collect data from 120 registered nurses employed in critical care and postcritical care units. Nurses provided information about motivational factors and intentions and a self-report of the proportion of time they followed guidelines. At least 2 weeks later, the nurses' hand hygiene performance was observed while they provided patient care. Structural equation modeling was used to test the TPB-based model. Rate of adherence to recommendations for 1248 hand hygiene indications was 70%. The correlation between self-reported and observed adherence to handwashing recommendations was low (r = 0.21). TPB variables predicted intention to handwash, and intention was related to self-reported hand hygiene. Intensity of activity in the nursing unit, rather than TPB variables, predicted observed adherence to hand hygiene recommendations. The limited association between self-reported and observed hand hygiene scores remains an enigma to be explained. Actual hand hygiene behavior may be more sensitive to the intensity of work activity in the clinical setting than to internal motivational factors.