Obesity and osteoporosis may have their origins in early postnatal life. This study was designed to evaluate whether flaxseed flour use during lactation period bears effect on body adiposity and skeletal structure of male rat pups at weaning. At birth, male Wistar rats were randomly assigned to control and experimental (FF) groups, whose dams were treated with control or flaxseed flour diet, respectively, during lactation. At 21 days of age, pups were weaned to assess body mass, length and composition by dual-energy X-ray absorptiometry. The animals were then sacrificed to carry out analysis of serum profile, intra-abdominal adipocyte morphology and femur characteristics. Differences were considered significant when P<0.05. The FF group displayed the following characteristics (P<0.05): higher body mass, length, bone mineral content, bone area and concentrations of osteoprotegerin, osteocalcin and high-density lipoprotein cholesterol; higher levels of stearic, α-linolenic, eicosapentaenoic and docosapentaenoic acids and lower levels of arachidonic acid and cholesterol; smaller adipocyte area; and higher mass, epiphysis distance, diaphysis width, maximal load, break load, resilience and stiffness of femur. Flaxseed flour intake during lactation period promoted adipocyte hypertrophy down-regulation and contributed to pup bone quality at weaning.