The prevalence and correlates of jitteriness were evaluated in a sample of 936 healthy full-term infants. Jitteriness was seen in 44% of this sample: 23% were classified as mildly jittery, 8% as moderately jittery, and 13% as extremely jittery. Jitteriness was seen most commonly in infants who were sleepy or active and least commonly in infants who were quietly wakeful during the neonatal examination. Jittery infants were more likely to be difficult to console when crying (P less than .01) and less visually alert (P less than .001) than were nonjittery infants. Jitteriness was seen more commonly in slightly smaller (P less than .05) and shorter (P less than .001) infants, in those more than 12 hours old (P less than .01), and in those not exposed to general anesthesia (P less than .05). In an expanded sample of 1054 healthy and sick full-term infants, jitteriness was observed more commonly in neonates who had been exposed prenatally to maternal marijuana use (P less than .01), but not to cocaine use (P = .1), and whose mothers had a positive postpartum urine assay for marijuana (P less than .05) or cocaine (P = .06). The magnitude of these drug effects, however, was small. These findings have important implications for the early parent-infant relationship, pediatric practice, and future research.