The impact of influenza on work absenteeism is poorly documented. We used data from the national registry and Norway Post AS (>14,000 employees) to explore sickness absence patterns from 2005/06 through 2009/10 in Norway. Annually, an estimated 2.868% (mean 95% confidence interval (CI): 2.405-4.820%) of the working population obtained sick leave for influenza, of whom 0.915% (mean 95% CI: 0.453-1.590%) had diagnoses for other respiratory illnesses with influenza as underlying cause. In the 2009/10 pandemic season, the absence rate increased 1.5-fold, mainly due to a 73% increase in influenza-diagnosed sick leaves. At Norway Post AS, absence related to seasonal influenza accounted for 0.351% (mean 95% CI: 0.126-0.704%) of total person-days annually (excluding parental care absence), of which 32-43% were estimated to be self-certified. Medically certified sick leave increased 1.3-fold in the pandemic season to 0.458% (95% CI: 0.176-0.856), while self-certified sick leave remained at a level typical for seasonal influenza. We found a significant four-fold increase in work loss to care for sick children, 0.048% (95% CI: 0.031-0.070%) of person-days, compared with 0.012% (95% CI: 0.004-0.028%) in 2008/09. In conclusion, GP-certified and parental care work absence were higher in the pandemic season. More studies are needed to quantify the burden of self-certified sick leave.