The mortality experience of a cohort of 3,827 white men licensed to apply pesticides in Florida was evaluated to investigate health effects associated with chronic exposure to pesticides. Although the overall standardized mortality ratio (SMR) for these structural pest control workers was not significantly elevated (SMR = 103), excess deaths were observed for leukemia, particularly acute myeloid leukemia (3 observed vs. 0.9 expected), and cancers of the brain (SMR = 200) and lung (SMR = 135). The risk of lung cancer rose with the number of years licensed with SMR of 101, 155, and 289 among those licensed for less than 10 years, for 10-19 years, and for 20 years or more, respectively. Ratios of directly adjusted rates showed similar patterns with observed-to-expected ratios of 100, 175, and 186 for the length of licensure categories. Mortality from lung cancer was greater among persons first licensed before age 40 (SMR = 234) than among those first licensed after age 40 (SMR = 115). Although information on tobacco use was not available, the increasing risk of lung cancer with number of years licensed and the capacity of certain pesticides to produce neoplasms in laboratory animals suggested that some pesticides may be carcinogenic in humans.