OBJECTIVES: Laboratory monitoring of patients using lithium is important to prevent harm and to increase effectiveness. The aim of this study is to determine compliance with the guidelines for laboratory monitoring of patients treated with lithiumverall and within subgroups. METHODS: Patients having at least one lithium dispensing for six months or longer between January 2010 and December 2015 were identified retrospectively using data from the Dutch PHARMO Database Network. Laboratory monitoringwas defined as being compliant with the Dutch Multidisciplinary Clinical Guideline Bipolar Disorders when lithium serum levels, creatinine and thyroid-stimulating hormone (TSH) had been measured at least every six months during lithium use. RESULTS: Data were analyzed from 1,583 patients with a median duration of seven six-month periods of lithium use. Results indicated that patients had been monitored over six-month periods for lithium serum levels 65% of the time, for creatinine 73% of the time and for TSH 54% of the time. Just over one seventh (16%) of patients had been monitored in compliance with the guidelines for all three parameters during total follow-up. Especially males, patients aged below 65 years, patients receiving prescriptions solelyfrom general practitioners, prevalent users of lithium, patients without interacting co-medication, and patients without other days with laboratory measurements had been monitored less frequently in compliance with the guidelines. CONCLUSION: A considerable proportion of patients had not been monitored in accordance with the guidelines. Further research is needed to understand the reasons for noncompliance and to implement strategies with the ultimate goal of optimizing safety and effectiveness for patients treated with lithium. This article is protected by copyright. All rights reserved.