Objective. To determine the reliability and validity of a modified version of the Fibromyalgia Impact Questionnaire (FIQ) in evaluating patients with post-Lyme disease syndrome (PLDS). Methods. In this cross-sectional analysis 13 PLDS, 18 fibromyalgia (FM), and 16 healthy controls (n 47) completed a modified FIQ containing items to evaluate physical impairment, symptom severity, and global well-being. Comparisons between groups were done using analysis of variance with a significance level set at 0.05. Results. PLDS patients demonstrated statistically significantly greater levels of impairment than controls in physical functioning, FIQ total score, global well-being, joint pain, fatigue, depression, ability to perform activities of daily living, and memory/con-centration. FM patients demonstrated a statistically significantly greater level of impairment than the control group in all categories, and the scores were significantly higher than the PLDS group in the measurement of physical impairment, FIQ total score, muscle pain, and joint pain. Overall, the instrument possesses good reliability and validity, although adequacy of this instrument to measure impairment in the male PLDS population needs further elucidation. Conclusion. The results of this study suggest that the modified FIQ may be a useful tool in evaluating PLDS patients. The findings suggest that there may be some differences in the etiopathology of the symptoms experienced by PLDS and FM patients.