AIMS--This study aimed to characterise the status of viral infection in patients with HTLV-1 uveitis (HU) by quantifying the circulating HTLV-1 infected cells in the peripheral blood. METHODS--Genomic DNA samples of peripheral blood mononuclear cells (PBMC) were obtained from 25 patients with HU, 14 patients with tropical spastic paraparesis/HTLV-1 associated myelopathy (TSP/HAM), and 21 asymptomatic carriers of HTLV-1. Quantitative polymerase chain reaction (PCR) of the gag region of HTLV-1 provirus DNA was performed on these DNA samples. To confirm the PCR, genomic Southern blot hybridisation was performed to identify integrated HTLV-1 provirus. This procedure detected a few percent of HTLV-1 infected cells in the PBMC. RESULTS--Most of the HU patients had a significantly increased number of circulating HTLV-1 infected cells (mean (SD) 3.84% (4.45%) of the PBMC), whereas the percentage of infected cells in most asymptomatic carriers was less than 1% (0.54% (1.11%)). Most of the TSP/HAM patients also had a relatively high percentage (11.63% (7.67%)). The differences among these three groups were highly significant by the Mann-Whitney U test. CONCLUSION--The results suggested that the increase in the number of HTLV-1 infected cells is one base for the development of inflammatory HU lesions, as it is for TSP/HAM.