Procalcitonin (PCT) measurement has been proposed to direct antibiotic use. We examined whether repeated PCT measurements (0, 6, and/or 12 hours) versus the initial measurement only (time 0) increased the sensitivity and specificity of PCT for diagnosing infection in intensive care unit patients. Infection was identified in 67/176 (38%) patients. The sensitivity of repeated versus the initial PCT measurement (with a cutoff value 0.5 ng/mL) was 52/67 (77%; 95% confidence interval [CI], 66%-87%) vs 46/67 (69%; 95% CI, 56%-79%; P = .04) and specificity 60/109 (55%; 95% CI, 45%-65%) vs 59/109 (54%; 95% CI, 44%-64%; P = 1.0). Repeat PCT evaluations over 12 hours did not provide a clinically significant improvement in diagnostic accuracy when compared to the initial single test.