Objectives
In Japan, a rubella outbreak occurred from early 2012 to late 2013, primarily among adult males aged 20–49 years. We conducted this study to predict the number of congenital rubella syndrome (CRS) cases in Japan in 2014.

Methods
The probability of CRS when a pregnant woman is infected with rubella depends on the gestational age of the fetus. The cumulative number of CRS cases was predicted by a formula based on the parameters from two studies conducted in the U.K. and the U.S., the reported cases of rubella among women 15–49 years of age, and the reports of CRS from 2011 to week 2 of 2014.

Findings
While the predicted number of cases of CRS based on parameters from the U.K. study demonstrated a biphasic curve, with a low peak around week 12 and a high peak around week 50 of 2013, the predicted number of CRS cases based on the U.S. study demonstrated a single peak around week 50 of 2013. The ex post evaluation indicated that the cumulative number of CRS cases in 2014 would be 19.1–29.3.

Interpretation
Our prediction of the number of CRS cases may be useful for the enhanced detection of this often under-reported syndrome.