TDP-43, encoded by TARDBP, is a ubiquitously expressed, primarily nuclear protein. In recent years, TDP-43 has been identified as the major pathological protein in ALS due to its mislocalisation in the cytoplasm of motor neurons of patients with and without TARDBP mutations and expression in forms that do not match its predicted molecular weight. In this study, the TDP-43 profile was investigated using western immunoblot analysis in whole lysates, nuclei and cytoplasm of circulating lymphomonocytes from 16 ALS patients, 4 with (ALS/TDP+) and 12 without (ALS/TDP-) TARDBP mutations in the protein C-terminal domain, and thirteen age-matched, healthy donors (controls). Three disease-unaffected first-degree relatives of an ALS/TDP+ patient were also included: one carried the parent mutation (Rel/TDP+) whereas the other two did not (Rel/TDP-). In all ALS patients, relatives and controls, TDP-43 retained the predicted molecular weight in whole cell lysates and nuclei, but in the cytoplasm its molecular weight was slightly smaller than expected. In quantitative terms, TDP-43 was expressed at approximately the same levels in whole cell lysates of ALS patients, relatives and controls. In contrast, TDP-43 accumulated in the cytoplasm with concomitant nuclear depletion in all ALS/TDP+ patients, in about 50% of ALS/TDP- patients and in the Rel/TDP+ subject compared to the controls. In the remaining ALS/TDP- patients and in the two Rel/TDP- subjects, TDP-43 matched the control levels in both subcellular compartments. Were these findings further confirmed, circulating lymphomonocytes could be informative of TDP-43 mislocalisation in nervous tissue and used as a biomarker for future disease risk.