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Objectives: Permethrin use has been associated with an increased risk of multiple myeloma (MM) among pesticide applicators. However, the biological plausibility and mechanisms underlying this association are not fully understood. The aim of this study was to assess whether exposure to permethrin is related to haematological alterations among occupationally exposed pesticide applicators., Methods: We conducted a longitudinal study among 33 pesticide applicators in the Biomarkers of Exposure and Effect in Agriculture study comparing haematological parameters in the offseason with the day after permethrin exposure and, for 27 participants, approximately 3 weeks postexposure. Complete blood counts with white blood cell differential and lymphocyte subsets were measured at each visit. Multivariate linear mixed effects models were used to assess the relationship between natural log-transformed haematological parameters and exposure to permethrin., Results: The adjusted geometric mean immature granulocyte count was elevated among pesticide applicators following permethrin exposure compared with their offseason levels (37% increase, 95% CI 6% to 76%). Modest but statistically significant (p<0.05) alterations in red blood cell (RBC) parameters (eg, decreased RBC count and haemoglobin and increased mean corpuscular volume and RBC distribution width-SD) were also observed the day after permethrin use compared with offseason levels; decreases in RBC count and haemoglobin and increases in RBC distribution width-SD persisted approximately 3 weeks after permethrin use., Conclusions: Altered haematological parameters could be indicative of disrupted haematopoiesis, providing insights into the biological plausibility of the observed association between permethrin use and MM risk among pesticide applicators., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2019. No commercial re-use. See rights and permissions. Published by BMJ.)