Little is known about seasonality and long-term trends in the incidence of pediculoses. Such information would help both public health workers and clinicians in the prevention and management of the problem. In the Israel Defence Forces the pediculoses are individually notifiable, and this provides a unique database for the study of this subject. The incidence of head louse infestation increased from 5.2 per 1000 in 1977 to 18.1 per 1000 in 1979, and then declined to 3.6 per 1000 in 1987. The incidence was higher in the warmer months. Pubic louse infestation increased from 7 per 1000 in 1977 to 14.9 per 1000 in 1983, and then declined to 4.6 per 1000 in 1987. The incidence was higher in the cooler months. Possible causes of the seasonality and long-term trends are discussed.