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Patients with primary hematological malignancy (HM) are at an elevated risk of subsequent malignant neoplasms (SMNs), which is a common concern after treatment of primary cancer. We identified 45,533 patients aged ≥20 years and diagnosed with primary HM in Finland from 1992 to 2019 from the Finnish Cancer Registry and estimated standardized incidence ratios (SIR) and excess absolute risks per 1000 person‐years (EAR) for SMNs. A total of 6076 SMNs were found (4604 solid and 1472 hematological SMNs). The SIRs were higher for hematological SMNs (SIR 4.9, 95% confidence interval [CI] 4.7–5.2) compared to solid SMNs (SIR 1.5, 95% CI 1.4–1.5). The SIRs for hematological SMNs were highest in the young HM patients aged 20–39 years (SIR 9.2, 95% CI 6.8–12.2 in males and SIR 10.5, 95% CI 7.2–14.7 in females) and decreased by age of first primary HM. However, EARs for hematological SMNs were highest in the older patients, aged 60–79 years at their first primary HM (EAR 5.7/1000 and 4.7/1000 in male and female patients, respectively). In conclusion, the incidence of both hematological and solid SMNs were increased in hematological cancer patients. The relative risk (SIR) was highest among younger HM patients with hematological SMNs. The absolute second cancer burden reflected by high EAR arises from solid malignancies in older patients. Our results accentuate the need for vigilance in the surveillance of HM patients. [ABSTRACT FROM AUTHOR]