Background: Hodgkin lymphoma is potentially curable. However, 15–35% of older patients (ie, >60 years) have a lower response rate, worse survival outcomes, and greater toxicity than younger patients. Brentuximab vedotin and nivolumab exhibit activity in patients with relapsed or refractory Hodgkin lymphoma. We therefore aimed to evaluate the safety and efficacy of brentuximab vedotin and nivolumab in untreated older patients with Hodgkin lymphoma or in younger patients considered unsuitable for standard ABVD (ie, doxorubicin, bleomycin, vinblastine, and dacarbazine) therapy. Methods: We did a multicentre, single-arm, phase 2 trial at eight cancer centres in the USA. Previously untreated patients with classic Hodgkin lymphoma were eligible for study enrolment if they were 60 years or older, or younger than 60 years but considered unsuitable for standard chemotherapy because of a cardiac ejection fraction of less than 50%, pulmonary diffusion capacity of less than 80%, or a creatinine clearance of 30 mL/min or more but less than 60 mL/min, or those who refused chemotherapy. Patients were also required to have an Eastern Cooperative Oncology Group (ECOG) performance status of 0–2. Patients received brentuximab vedotin at 1·8 mg/kg (dose cap at 180 mg) and nivolumab at 3 mg/kg both intravenously every 21 days for 8 cycles. The primary endpoint was the overall response, defined as a partial metabolic response or complete metabolic response at the end of 8 cycles of treatment. A per protocol analysis was done including all patients who received treatment in the activity and safety analyses. This study is registered with ClinicalTrials.gov, number NCT02758717. Findings: Between May 13, 2016, and Jan 30, 2019, the study accrued 46 patients. The median age was 71·5 years (IQR 64–77), with two (4%) of 46 patients younger than 60 years. Median follow-up was 21·2 months (IQR 15·6–29·9), and 35 (76%) of 46 patients completed all 8 cycles of therapy. At the interim analysis on Oct 11, 2019, the first 25 evaluable patients had an overall response rate of 64% ([95% CI 43–82] 16 of 25 patients; 13 [52%] had a complete metabolic response and three [12%] had a partial metabolic response). The trial was closed to accrual on Oct 14, 2019, after the interim analysis failed to meet the predefined criteria. In all 46 evaluable patients, 22 (48%) patients achieved a complete metabolic response and six (13%) achieved a partial metabolic response (overall response rate 61% [95% CI 45–75]). 14 (30%) of 46 patients had 16 dose adjustments, primarily due to neurotoxicity. 22 (48%) of 46 patients had peripheral neuropathy (five [11%] patients had grade 3 peripheral neuropathy). Grade 4 adverse events included increased aminotranferases (one [2%] of 46), increased lipase or amylase (two [4%]), and pancreatitis (one [2%]). One (2%) patient died from cardiac arrest, possibly treatment related. Interpretation: Although the trial did not meet the prespecified activity criteria, brentuximab vedotin plus nivolumab is active in older patients with previously untreated Hodgkin lymphoma with comorbidities. The regimen was also well tolerated in the majority of patients in this older population. Future trials should be based on optimising the dose and schedule, perhaps combined with other targeted agents that might permit chemotherapy-free strategies in older patients with Hodgkin lymphoma. Funding: Seattle Genetics and Bristol Myers Squibb.