Radiotherapy (RT) near salivary glands results in changes in the oral flora in favour of cariogenic organisms and an increased susceptibility to caries. The aim of this study was to assess the effect of a 12-month preventive programme on caries incidence and on the levels of mutans streptococci (ms) and lactobacilli in tongue loop samples taken from patients before, during and after RT. The regime consisted of 2 x daily rinsing with 10 ml 0.2% chlorhexidine, diluted 1:1 with water, for 1 week before RT, during RT and for 4 weeks after RT. This was then substituted with a 0.05% NaF rinse daily. A saliva substitute containing 2 ppm F was used as required. Scaling was carried out before RT and dietary advice and oral hygiene instruction given. Appropriate radiographs were taken at baseline and after 6 and 12 months. Tongue loop samples for microbiology were taken in the middle and end of RT and subsequently at 6, 8, 12, 24, 40 and 52 weeks. Whenever levels of ms exceeded 2 x 10(5) cfu/ml sample, 1% chlorhexidine gel in custom-made applicator trays was applied by the subject for 5 min daily for 14 days. In 25 subjects completing the programme, there was a total of 3 new caries lesions after 12 months. Thirteen pre-existing enamel lesions were arrested. There were significant reductions (p < 0.005) in ms levels from baseline values during RT and 4 weeks after RT. There were no significant increases in ms levels throughout the study.(ABSTRACT TRUNCATED AT 250 WORDS)