1. We studied perforin gene expression in T lymphocytes obtained from 26 patients with IgA nephropathy and from 15 healthy age-matched control subjects. 2. The majority of patients with IgA nephropathy (96%) had elevated perforin mRNA expression, whereas no perforin mRNA expression was detected in the T lymphocytes of normal control subjects. 3. A positive correlation was noted between perforin mRNA expression and urinary protein excretion. 4. Perforin mRNA expression correlated also with the histopathology in the renal tissue of patients with IgA nephropathy. 5. Sixty per cent of patients with grade III or IV histopathology had high perforin mRNA expression in T lymphocytes [more than (++)]. 6. These studies suggest that disregulation of perforin gene expression in T lymphocytes may be associated with the progression of IgA nephropathy and could be used as an indicator of disease activity.