The human endogenous retrovirus HERV-K18 is located within intron 1 of CD48 on chromosome 1q and is still active in the human genome. Genetic variation in HERV-K18 single-nucleotide polymorphisms (SNPs) has previously been associated with an increased risk of schizophrenia (SZ) and with type 2 diabetes (T2D) among individuals with SZ. Here, we present a replication study of association of two SNPs in HERV-K18 and 19 tagSNPs in CD48 with (a) SZ and (b) T2D in patients with SZ in two Danish samples (total number of cases=750 and controls=1214). No association was found with SZ or with T2D among individuals with SZ for any of the investigated SNPs. However, one HERV-K18 SNP showed a tendency toward an association with T2D in younger SZ patients, in agreement with previous findings, but due to a very low sample size, this result needs to be further investigated.