IMPORTANCE: Epilepsy affects approximately 65 million people worldwide, and 60% have focal seizures. Predicting seizure response and drug resistance to antiseizure medications (ASMs) in people with focal epilepsy remains difficult. OBJECTIVE: To describe the expected short- and long-term response to treatment with ASMs in people with focal epilepsy using recognized definitions by the International League Against Epilepsy. DESIGN, SETTING, AND PARTICIPANTS: The Human Epilepsy Project is an international, prospective, observational cohort study that followed up people with newly diagnosed focal epilepsy for up to 6 years between 2012 and 2020. Data were analyzed from 2023 to 2024. The Human Epilepsy Project was conducted at 34 tertiary epilepsy centers across the US, Australia, and Europe. Participants with confirmed diagnosis of focal epilepsy aged 12 to 60 years were enrolled within 4 months of treatment initiation with ASM(s). Data were analyzed from February 2024 to July 2024. EXPOSURE: ASM (variable). MAIN OUTCOMES AND MEASURES: The primary outcome was seizure freedom, defined as a period without seizures for 12 months or 3 times the longest pretreatment seizure-free interval, whichever was longer. Treatment response was categorized as sensitive, meaning seizure free receiving 2 or fewer adequate ASM trials; resistant, meaning having 2 or more adequate ASM trials fail; or indeterminate (neither treatment sensitive nor resistant). RESULTS: Among 448 enrolled participants, 267 (59.6%) were female, and median (IQR) participant age was 32 (21-44) years at treatment initiation. Median (IQR) follow-up duration was 3.13 (2.33-3.55) years. Most achieved seizure freedom (267 participants of 448 [59.6%]), largely without relapse (223 [83.5%]). There were 245 treatment-sensitive participants (54.7%), 102 treatment-resistant participants (22.8%), and 101 indeterminate participants (22.5%). Among treatment-sensitive participants, most (217 [89.3%]) responded to monotherapy and half (121 [49.4%], or 27% of total cohort) became seizure free while receiving their first ASM. In the first year of treatment, 251 participants (63%) had ongoing or worsening seizures. Median time to first seizure freedom was 12.1 months (95% CI, 9.7-16.1). This occurred earlier in those who never relapsed (median, 2.2 months; 95% CI, 0.8-3.2) than those who did (median, 7.4 months; 95% CI, 4.0-10.7). Those with infrequent pretreatment seizures were 0.41-fold more likely to be treatment resistant than those with very frequent seizures (relative risk [RR], 0.41; 95% CI, 0.18-0.89; P = .03; HB-corrected P = .02). Participants with self-reported comorbid psychological disorders were 1.78-fold more likely to be treatment resistant than those without (RR, 1.78; 95% CI, 1.26-2.52; P = .001). CONCLUSIONS AND RELEVANCE: In the Human Epilepsy Project multicenter prospective cohort study, most people with newly diagnosed focal epilepsy took more than a year and more than 1 ASM to become seizure free. Drug resistance can be identified earlier in those with frequent pretreatment seizures, and a history of psychiatric comorbidities at epilepsy diagnosis is an important prognostic factor. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT02126774.