Natural measles infection usually confers life-long immunity which is transferred from mothers to their offspring, protecting them from natural measles until the age of about 12 months. Recently, however, natural measles has been observed with increased frequency in infants under the age of 12 months. Natural measles outbreaks in the city of Sapporo have been suppressed by widely applied measles vaccination. Passive measles immunity in 160 neonates (cord blood), born during the last 17 y in Sapporo, Japan was determined by a neutralization (NT) antibody test. The mothers of these infants had had natural measles infection during childhood. Geometric mean titres (GMTs) of cord blood NT antibodies gradually decreased after 1989 and the GMTs of the most recently born infants were significantly lower than those of infants born in the first few years of the study. These observations suggest that even in mothers who experienced natural measles in childhood, recurrent exposure to natural measles is necessary in order to maintain adequate antibody levels for effective passive immunity of their infants.