Hypoxemia has previously been reported during lumbar puncture (LP) in infancy. The purpose of this study was to determine whether preoxygenation before the LP would reduce hypoxemia during the procedure in infants. Twenty-one infants (one to 15 weeks of age) undergoing LP for evaluation of possible sepsis were randomly assigned to the control group (12) or treatment group (9). The treatment group was preoxygenated breathing oxygen (FiO2 = 1.0) spontaneously via snug face mask for three minutes prior to being positioned for the LP. The control group spontaneously breathed room air during this interval. Oxyhemoglobin saturation was measured prior to, and continuously during, the LP with pulse oximetry. The groups were comparable in age, resting respiratory rate, baseline saturation, and duration of the procedure. The treatment group developed significantly less desaturation during the procedure than the control group (P < 0.05). We conclude that preoxygenation prior to LP prevents most of the hypoxemia resulting from the procedure in infants.