This study uses 44 consecutive months of data from the Health Care Financing Administration to assess seasonal trends in hip fracture incidence among the United States white population aged 65 years and older. The authors studied a total of 621,387 cases of hip fracture which occurred from January 1984 to September 1987. During the study period, hip fracture incidence rates display a distinctive pattern of seasonal periodicity; high rates are found in the winter and low rates in the summer among both males and females. This pattern of seasonal periodicity is consistent at ages 65-74 years, 75-84 years, and greater than or equal to 85 years. When the time series of rates are stratified into five geographic levels, each level covering 5 degrees of latitude, the distinctive pattern of seasonal periodicity is the same for all levels.