OBJECTIVES: The economic impact of flu and flu-like illnesses (FFLIs) in Hong Kong has never been investigated. Such data will facilitate the development of a cost-effective health-care policy against the diseases. The objectives of the study include: 1) to determine the incidence and health effects of FFLIs, and 2) to estimate the cost impact of these in a big corporation in Hong Kong from an employer's perspective. METHODS: A questionnaire to study the 11,000 employees of a Hong Kong-based major international airline company was developed. The two major categories of employees surveyed were the ground staff and the flight crew due to different working environments and thus different risk factors for flu. The questionnaire captured information on the age, sex, years of service, grade and number of episodes in the past 12 months. Information about the nature and duration and of the effects on the well being of the individual during each episode was obtained. The need for medical treatment and days off was also documented. RESULTS: Eleven thousand questionnaires were sent out and 2,212 were returned (response rate 20.1%). Among the flight crew, each member of the cockpit and cabin crews was estimated to lose on average 14.4 and 13.0 "equivalent days of perfect health" (EDPH) per year respectively. For the ground staff, each member of the check-in and back-office staff was estimated to lose 6.9 EDPH and 8.7 EDPH per year respectively. In terms of productivity, depending on the salary-scale of the staff, between HK$2,321 (check-in staff), (1US$=7.8HK$), and HK$36,791 (cockpit crew) was lost per year per worker. Evidence indicated that a disproportionately large proportion of the flu episodes among aircrew occurred during or after a trip to Europe and S.E. Asia. CONCLUSION: The impact of FFLIs on flight crew appears to be greater than the impact on ground staff.