Investigations carried out to estimate the effect of long-term occupational exposure to low levels of external ionizing radiation indicated that exposed hospital staff showed an increase in chromosome aberrations. The purpose of this study was to evaluate whether genomic instability or an alteration in pteridine synthesis could be used as a marker of the potential hazard of ionizing radiation in hospital workers. Twenty gamma-radiation- and 33 X-ray-exposed technicians working in radiotherapy and radio-diagnostic units were included in this study, along with 22 healthy matched individuals. Plasma concentrations of nitrite plus nitrate (NO,) were measured to estimate reactive nitrogen species. Urinary neopterin, biopterin and creatinine concentrations were measured by high-performance liquid chromatography to determine metabolic activity along the pteridine pathway. Sister chromatid exchange was used as a measure of mutagenicity. Apoptosis was evaluated morphologically and also with a DNA-fragmentation test.