Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia/lymphoma (ATLL), HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), and other diseases. The mechanisms of virus pathogenesis are still obscure. The occurrence of defective proviruses in HTLV-1-infected cell lines and the peripheral blood mononuclear cells (PBMC) of infected individuals is a frequent feature of virus infection. We detected defective proviruses with large internal deletions in PBMC from ATLL and HAM/TSP patients and in asymptomatic HTLV-1 carriers. Seventeen PCR-amplified defective proviruses were sequenced, and three types of deletions were found. Besides truncated MA and the 5' end of the genome, truncated CA, truncated SU, and more frequently truncated TM linked to the pX region were detected. Reverse transcription-PCR analysis of PBMC from ATLL patients and asymptomatic carriers also revealed RNA transcripts with large internal deletions. Analysis of two RT-PCR cDNA clones confirmed a Gag-TM-pX structure of the transcripts. Most defective proviruses contained numerous internal stop codons, but some were capable of coding for the truncated MA linked to a variable out-of-frame peptide. Cloned defective proviruses with long open reading frames were subjected to in vitro transcription-translation followed by radioimmunoprecipitation, which showed expression of chimeric proteins between 8 and 12 kDa. Possible roles of defective proviruses and chimeric proteins are discussed, although there is no firm association with pathogenesis.