We assessed the toxicity and clinical outcomes associated with linezolid therapy (mean duration, 29+/-28 days; range, 8 to 185 days) in 44 patients with serious gram-positive infections. Although a clinical cure was achieved in 73% of the cases, 28/44 (64%) had adverse reactions (thrombocytopenia, n=13; anemia, n=7; gastrointestinal, n=12; peripheral neuropathy, n=1; serotonin syndrome, n=1), such that a systematic monitoring protocol was developed.