Background. Alexander Litvinenko died on November 23, 2006, from acute radiation sickness syndrome caused by ingestion of polonium-210 (210Po). Objective. The objective was to assess the prevalence of and risk factors for internal contamination with 210Po in healthcare workers (HCWs) caring for the contaminated patient. Setting. Hospital. participants. HCWs who had direct contact with the patient. Methods. We interviewed 43 HCWs and enquired about their activities and use of personal protective equipment (PPE). Internal contamination was defined as urinary 210Po excretion above 20 mBq within 24 hours. We obtained risk ratios (RRs) for internal contamination using Poisson regression. Results. Thirty-seven HCWs (86%) responded, and 8 (22%) showed evidence of internal contamination, all at very low levels that were unlikely to cause adverse health outcomes. Daily care of the patient (washing and toileting the patient) was the main risk factor (RR, 3.6 [95% confidence interval (CI), 1.1-11.6]). In contrast, planned invasive procedures were not associated with a higher risk. There was some evidence of a higher risk associated with handling blood samples (RR, 3.5 [95% CI, 0.8-15.6]) and changing urine bags and/or collecting urine samples (RR, 2.7 [95% CI, 0.8-9.5]). There was also some evidence that those who reported not always using standard PPE were at higher risk than were others (RR, 2.5 [95% CI, 0.8-8.1]). Conclusions. The sensitive quantitative measurement enabled us to identify factors associated with contamination, which by analogy to other conditions with similar transmission mechanisms may help improve protection and preparedness in staff dealing with an ill patient who experiences an unknown illness.