We reviewed major errors in Gram stain reports from positive blood cultures to identify patterns and potential clinical impact. During a 23-month period, blood cultures were misread for 57 (0.7%) of 8,253 patients. Of 5,885 read as gram-positive cocci, 6 (0.1%) had only gram-negative organisms by culture, 3 of which were Acinetobacter species Of 1,959 read as gram-negative bacilli, 25 (1.3%) had only gram-positive organisms by culture. Of these, 9 were Bacillus and 2 were Clostridium species. Nonrecognition of mixed Gram stains accounted for 28 errors that most often were associated with a reading of gram-positive cocci. In 4 cases, there were delays of 14 hours to 3 days in starting appropriate antibiotic treatment; 2 deaths occurred, although the erroneous Gram stain report probably was not contributory. Pathologists and laboratory personnel need to be aware of these types of misinterpretations and the potential effects on patient outcome.