Lithium administration can reportedly cause toxicity, including lithium-associated thrombosis; however, not all reported cases of this adverse effect have been attributable to lithium overdoses. We report here two cases of deep vein thrombosis that occurred in association with lithium toxicity. Lithium overdose was deemed to be the cause in only one of these cases; a patient in whom deep vein thrombosis occurred 11 days after identification of lithium toxicity. In the other patient, the deep vein thrombosis occurred 15 days after diagnosis of lithium toxicity; this patient was not considered to have been overdosed. Both patients had other risk factors in addition to receiving lithium. We recommend monitoring D-dimer concentrations to facilitate early detection of deep vein thrombosis in patients with lithium toxicity.