The objective of this study was to examine the usefulness of a mindfulness-based cognitive therapy (MBCT) for treating insomnia symptoms in patients with anxiety disorder. Nineteen patients with anxiety disorder were assigned to an 8-week MBCT clinical trial. Participants showed significant improvement in Pittsburgh Sleep Quality Index (Z = −3.46, p = 0.00), Penn State Worry Questionnaire (Z = −3.83, p = 0.00), Ruminative Response Scale (Z = −3.83, p = 0.00), Hamilton Anxiety Rating Scale (Z = −3.73, p = 0.00), and Hamilton Depression Rating Scale scores (Z = −3.06, p = 0.00) at the end of the 8-week program as compared with baseline. Multiple regression analysis showed that baseline Penn State Worry Questionnaire scores were associated with baseline Pittsburgh Sleep Quality Index scores. These findings suggest that MBCT can be effective at relieving insomnia symptoms by reducing worry associated sleep disturbances in patients with anxiety disorder. However, well-designed, randomized, controlled trials are needed to confirm our findings.