AbstractHuman herpesvirus‐8 (HHV‐8) viremia is associated with refractory conditions in patients infected with HIV‐1. Therefore, we evaluated the factors related to plasma HHV‐8‐DNA. Participants included patients infected with HIV‐1 who visited our hospital. Plasma HHV‐8‐DNA levels were measured using real‐time polymerase chain reaction, and anti‐HHV‐8 antibodies were assessed through enzyme immunoassays using multiple antigens (K8.1, ORF59, ORF65, and LANA). Factors related to plasma HHV‐8‐DNA were examined using Fisher's exact test or Mann–Whitney U test. The study involved 36 patients infected with HIV‐1, of whom 19 were histologically diagnosed with Kaposi's sarcoma (KS), two had multicentric Castleman's disease (MCD), and 15 did not exhibit HHV‐8‐related disease. Before the introduction of antiretroviral therapy (ART), plasma HHV‐8‐DNA was detected in 44% (7/16) of patients with KS and in 9% (1/11) of patients without HHV‐8‐related disease. Among patients with KS, elevated plasma HHV‐8‐DNA levels (≥0.05 copies/µL) correlated with the presence of CDC category C diseases other than KS (p = 0.0337), anti‐HHV‐8 antibody negativity (p = 0.0337), anemia (p = 0.0474), and thrombocytopenia (p = 0.0146). Following ART initiation, the percentage of patients positive for plasma HHV‐8‐DNA decreased from 44% (7/16) to 6% (1/17), and the percentage of patients positive for anti‐HHV‐8 antibodies increased from 44% (7/16) to 88% (15/17). Finally, plasma HHV‐8‐DNA positivity and anti‐HHV‐8 antibody negativity were observed in two patients with MCD. Our findings suggest that insufficient production of anti‐HHV‐8 antibodies was associated with HHV‐8 viremia, and that anti‐HHV‐8 antibody production was recovered with ART; thus, indicating the possibility of involvement of humoral immunity in suppressing HHV‐8 viremia.