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Background and Importance Relationship between circadian rhythm and anticancer immunotherapy has been studied in several solid tumours. Timing of infusion may influence treatment efficacy. One of the latest cancers treated with immunotherapy has been extensive-stage small-cell lung cancer (ES-SCLC).Aim and Objectives To assess whether the timing of atezolizumab infusions (early vs late) is associated with efficacy in ES-SCLC. The primary outcome was overall survival (OS). Secondary outcomes were progression-free survival (PFS), response, and immuno-related adverse events (IR-AEs).Material and Methods Retrospective, observational, descriptive study in a tertiary hospital. We collected data from patient who received atezolizumab and were diagnosed with ES-SCLC between 2021 and 2024. Data cutoff was in July 2025. Patient data were collected from electronic medical record (HCIS™). OS, PFS and response were determined by RECIST criteria; AE grade was determined using CTCAE v.5.0.Time-of-day infusion was divided before 15:00h (early-infusion) and after 15:00h (late-infusion). Patients were classified into each group if they received more than 50% of infusions in this time-of-day.Results The study included 83 SCLC-ES patients (men 59.0%; PS0–1, 72.3%), aged 47–84 years. Fourty patients received atezolizumab administrations before 15:00 (early-infusion group; PS0–1, 80.0%; brain and/or liver metastases 62.5%) and 43 after 15:00 (late-infusion group; PS0–1, 65.1%; brain and/or liver metastases 65.1%). Median PFS (95% CI) was 4.8 months (3.3–6.2) for early-infusion group and 3.9 months (2.9–4.9) for late-infusion one (p=0.056). Median OS was 7.8 months (4.6-10.9) and 6.7 months (3.9–9.4) for early-infusion and late-infusion group, respectively (p=0.842). Five patients (12.5%) have progression disease (PD) as response in early-infusion group, 32 (80.0%) partial response (PR) and 3 (7.5%) stable disease (SD); 12 (27.9%) patients have PD in late-infusion group, 21 (48.8%) RP and 10 (23.3%) SD (p=0.082).Immune-related AE were more frequent in early-infusion group (n=10; 25.0%) than in late-infusion one (n=3; 7.0%) p=0.003. Six patients had IR-AEs grade 3-4 in early-infusion group. In late-infusion one, all IR-AEs were grade 1-2.Conclusion and Relevance Early (before 15:00) atezolizumab infusions showed a numerical trend towards improved efficacy outcomes compared with late-infusions, but differences did not reach statistical significance. Furthermore, early-infusions were associated with a higher frequency and severe IR-AEs.Conflict of Interest No conflict of interest