The characteristics of cerebrospinal fluid (CSF) associated with traumatic lumbar puncture, defined as CSF red blood cell (RBC) count greater than 1000/mm3, were reviewed in 92 previously healthy children greater than 1 month of age; 30 had bacterial meningitis and 62 had negative CSF cultures. The purpose was to distinguish CSF profiles of the two groups despite contamination with peripheral blood elements. In each case, white blood cell (WBC) counts were observed (O) and compared with those predicted (P), calculated as P = CSF RBC X (blood WBC/blood RBC). Comparison of O:P ratios revealed that all 30 patients with bacterial meningitis had ratios greater than or equal to 1, 28 (93%) had ratios greater than 10, and 24 (80%) had ratios greater than 100; by contrast, only 2 patients (3%) with culture-negative CSF had ratios greater than 10, 21 (34%) had ratios of 1-10, and 39 (63%) had ratios less than 1. Significant differences were observed in the rate of O:P ratio greater than or equal to 1 (100% vs. 32%), CSF differential cell count predominance of polymorphonuclear leukocytes (97% vs. 11%), hypoglycorrhachia (73% vs. 3%), and positive Gram's-stained smear for pathologic organisms (80% vs. 0) in those with and without bacterial meningitis, respectively (P less than .0001). Thus, in children greater than 1 month of age, CSF abnormalities associated with bacterial meningitis are rarely obscured by blood contamination from traumatic lumbar puncture.