In cases of criminal thallium poisoning, forensic investigation is required to identify the amount and time of thallium exposure. Usually, blood and urine thallium levels are respectively used as biomarkers. Additionally, hair has the unique potential to reveal retrospective information. Although several studies have attempted to clarify how thallium is distributed in hair after thallium poisoning, none have evaluated the time course of changing thallium distribution. We investigated changes in the distribution of thallium in hair at different time points after exposure in five criminal thallotoxicosis patients. Scalp hair samples were collected twice, at 2.6 and 4.2-4.5months after an exposure incident by police. Results of our segmented analysis, a considerable amount of thallium was detected in almost all hair sample segments. The thallium exposure date estimated from both hair sample collections matched the actual exposure date. We found that determination of thallium amounts in hair samples divided into consecutive segments provides valuable information about exposure period even if a considerable time passes after exposure. Moreover, when estimating the amount of thallium exposure from a scalp hair sample, it is necessary to pay sufficient attention to individual differences in its decrease from hair.