Background
The risk of cancer among relatives of patients with either myelodysplastic neoplasia (MDS), acute myeloid leukemia (AML) or acute lymphoblastic leukemia (ALL) has not been thoroughly examined.

Methods
We linked the Danish Civil Registration System with the Danish Cancer Registry, the Danish National Acute Leukemia Registry, and the Danish Myelodysplastic Syndrome Database to estimate the relative risk of cancer among relatives of patients with MDS/AML/ALL. We used standardized incidence ratios (SIRs), i.e., the ratio of observed to expected number of cancers among the relatives as a measure of relative risk.

Results
We identified 13010 first-degree (FDR) and 22051 second‐degree (SDR) relatives of 8386 patients with MDS/ALL/AML. Disregarding basal cell carcinoma (BCC), the relative risk for cancer overall was increased in both FDR (SIR=1.3; 95% confidence interval (CI) 1.1–1.4) and SDR (SIR=1.5; 95% CI 1.2–1.8). SIRs among FDRs were statistically significantly increased for malignant melanoma, BCC and for the combined groups of cancers of the male genital organs, urinary tract, and MDS/AML/ALL. Among SDRs, SIRs were statistically significantly increased for malignant melanoma, BCC, and cancers in the digestive organs and peritoneum.

Conclusions
We observed an increased risk of cancer among FDR and SDR of patients with MDS/AML/ALL.