IntroductionLorlatinib is a potent, brain penetrant, next-generation ALK/ROS1 TKI, with high response rates and durable responses, including the brain. However, a significant drawback is the manifestation of neurocognitive adverse events (NCAEs). Despite being generally low-grade in severity, these NCAEs can be physically and mentally disabling. Extensive neurocognitive testing in this group of patients is lacking; therefore we conducted this study. Patients and methodsThis observational prospective study was conducted across 3 Dutch university hospitals. Patients with metastatic NSCLC with an ALK- or ROS1-rearrangement and having an indication to start lorlatinib in daily clinical practice were eligible. The primary endpoints were to identify changes in neurocognitive functioning, measured through neurocognitive assessment at intervals of 2 weeks and 2 months after starting lorlatinib, in comparison to baseline. As a secondary endpoint, the correlation between neurocognitive impairment and self-reported neurocognitive dysfunction was examined. ResultsBetween June 2019 and October 2022, 22 patients were included. Among the various neurocognitive tests administered, only the Hopkins Verbal Learning Test-Revised parts b and c demonstrated a significant and clinically relevant decrease in scoring 2 weeks post initiation of lorlatinib (P = .036 and P = .003, respectively). However, these returned to baseline at the 2-month evaluation. The questionnaires did not result in significantly different outcomes over time. ConclusionLorlatinib treatment did not result in a sustained and significant decline within any of the specified neurocognitive domains.