Zoster immunoglobulin (ZIG) was given for prophylaxis to 95 neonates born to mothers with perinatal varicella. The treatment had no influence on the clinical attack rate; 48 (50%) of the children developed varicella. However, the ZIG treatment clearly influenced the course of the disease for newborns at particular risk, i.e. when maternal varicella developed within 4 days before and 2 days after delivery. Of 41 such neonates, 21 (51%) contracted varicella with an incubation mean time of 11 days. 13 of the 21 developed a very mild chickenpox (no fever, less than or equal to 20 pocks), 6 had a mild to normal disease, and 2 (10%) had more severe infections; none died or got sequelae after the disease. These results should be compared with the expected rate of complications in non-treated neonates in the defined risk group, where the mortality among those contracting varicella has been reported to be as high as about 30%.