Purpose: The aim of the present study has been the evaluation of the incomplete chromosome aberrations induced after α-particle irradiation by the simultaneous detection of all centromeres and telomeres present in human lymphocytes. Moreover, a study on the lengths of the different acentric fragments is presented. Materials and methods: Attached lymphocytes were irradiated at doses of 0.2, 0.5, 0.7 and 1 Gy using a 241Am source. Flourescent in-situ hybridization (FISH) techniques were applied using pan-centromeric and pan-telomeric probes. All abnormal cells were digitalised and analysed using a Cytovision FISH workstation. The description of all abnormalities observed, and the length of the acentric fragments was recorded. Results: A total of 378 incomplete chromosomes plus incomplete acentrics was found. Cases with more than 92 telomeres were not detected. The ratio between total incomplete elements and multicentrics was 1.00. The total number of acentric (ace) fragments was 822; 57% of them were complete fragments ace (+, +), 26% incomplete fragments ace (+, -), and 17% interstitial fragments ace(-,-); the mean relative lengths were 2.91 ± 0.06, 1.91 " 0.07 and 1.63 + 0.07, respectively. In all three cases a secondary peak in the length distribution was found, corresponding to a relative length between 3.5 and 4. Conclusion: The percentage of incomplete rejoinings is higher after α-particle exposure than that described previously for low-linear energy transfer (LET) radiation exposures. The results seem to indicate that compared to low-LET radiation, after α-particle exposure centromere-containing elements are more likely to be repaired. Many interstitial fragments are large linear forms that cannot be considered as non-distinguishable acentric rings.