Postoperative fever occurs in many patients. If no infection is found, atelectasis, if present, may be blamed. This study of 100 postoperative cardiac surgery patients followed up from day of surgery through the second postoperative day with daily portable chest radiographs and continuous bladder thermometry was designed to look for an association between atelectasis and fever. The daily incidence of atelectasis increased from 43 to 69 to 79%. However, the incidence of fever, defined as temperature ≥38.0°C fell from 37 to 21 to 17%. When defined as temperature ≥38.5°C, the daily incidence of fever fell daily from 14 to 3 to 1%. Using χ2 analysis, no association could be found between fever and amount of atelectasis. This contradicts common textbook dogma but agrees with previous human study and animal experiments.