Possible cytogenetic effects of theophylline have been investigated in asthmatic patients undergoing continuous therapy with this drug. Sister-chromatid exchanges (SCE), chromosome aberrations (CA) and proliferating rate indices (PRI) were evaluated in cultured peripheral blood lymphocytes from patients receiving theophylline alone, theophylline plus inhaled beta 2 adrenergic drugs or theophylline in combination with beta 2 adrenergic agents and corticoids. Two samples from each individual were obtained in order to perform a prospective study: before the theophylline medication (sample A) and at a time after the beginning of treatment (sample B). After treatment (66.3 +/- 37.8 days), an increase in SCE was observed without modifications either in PRI or in CA. Patients receiving beta 2 adrenergics or beta 2 adrenergics plus glucocorticoids before and during theophylline treatment, did not respond differently than those on theophylline alone.