Premature infants less than or equal to 34 weeks gestational age at birth and begun on ZDV by their clinical caregivers will be entered into the protocol within the first 5 days of life. Upon entry, ZDV will be administered every 12 hours intravenously or orally. The study will require 9 serum samples for ZDV assay collected over 30 days, some additional clinical laboratory tests collected over 12 weeks, collection of 3 spot urine samples, and collection of routine clinical data.