Introduction The 2009 A/H1N1 influenza spread also in Japan. Many students were absent in elementary schools, To prevent its expansion, many school decided class closure with their original length under rough guideline by local education board. However, its effect had not been clear. The objective was to examine whether or not the class closure length related to the change of absentees.

Methods Subject was all the classes in elementary schools in T city that class closure was carried in the period from 1 September to 24 December in 2009. We sent the questionnaire including the questions (1) the number of students in class, (2) the number of absentees on the day, (3) whether or not class closure was carried out on the day, to the school principal, and asked school nurses to write under each class attendance book and to return them by post mail. The length of class closure and the change of absentees before and after class closure was analysed by χ² test with statistical soft R2.11.1.

Results 16 of total 37 elementary schools replied (43.2%), and 103 classes of 15 schools with the closure were analysed. It revealed statistical associations between class closure length and the change of the proportions of absentee (p<0.001), and between class closure length and the proportions of class that absentee was decreased (p<0.001).

Conclusion There were tendencies that the number of absentee decreased after class closure, and the longer class closure days, the fewer absentees changed.