Introduction: Human T-lymphotropic virus 1 (HTLV-1) is related to several clinical diseases, including adult T-cell leukemia/lymphoma and HTLV-1-associated myelopathy. Intrafamilial transmission of HTLV-1 is rarely reported in Brazil and worldwide. Objective: identify HTLV-1 polymorphisms in family transmissions in Belem, Para, Amazon region of Brazil. Methods: Nested PCR of the pX gene, followed by Restrict Fragment Length Polymorphism (RFLP) to identify HTLV-1. Nested PCR of the 5’LTR region was used for nucleotide sequencing. Nucleotide sequences were analyzed using GENEIOUS 4.8.5 and MAFFT v.7. Results: The study investigated 72 individuals (14 index cases and 58 relatives) from 14 families, identifying HTLV-1 infection in 47% (27/58) of the relatives. Vertical transmission occurred in 14 (39%) of the 36 mother/child relationships investigated, and sexual transmission was observed in 14 (74%) of the 19 marital relationships. Both transmission routes were identified only in families with the highest average number of relatives investigated. Conclusion: Nucleotide analysis demonstrated HTLV-1 stability in intrafamilial transmission. High rate of HTLV-1 intrafamilial transmission was identified in this study, indicating infection perpetuation and virus stability.