In international comparisons or time trend analyses of cancer survival, it is common practice to restrict analyses to patients with a first cancer, i.e., to exclude patients with previous cancer diagnoses. However, the proportion of cancer patients with known previous cancer depends on the time cancer registries have been running, which results in varying proportions of excluded patients across registries. If prognosis of patients with second cancers differs from prognosis of patients with first cancers, varying exclusions may bias survival comparisons. We empirically evaluate the dependence of proportions of patients recorded as having a first cancer on time since initiation of cancer registration and the impact of excluding patients with known previous cancer on cancer survival estimates using the data of the nationwide Finnish Cancer Registry. Among 20 common cancer sites investigated, the proportion of “first cancers” varied between 97.4 and 99.7% in 1953–1957, the first 5-years of cancer registration, and decreased continuously to levels between 83.9 and 92.7% in 1993–1997. Excluding patients with a previous cancer diagnosis had little impact on estimates of survival of cancer patients diagnosed in 1953–1957, but increased 5-year relative survival estimates among patients diagnosed in 1993–1997 for each of the 20 cancers. The extent of the increase varied by cancer site and age. The increase ranged up to 2.9% points for crude and up to 1.7% points for age adjusted 5-year relative survival. These results caution against exclusion of patients with previous cancer diagnosis in comparative analyses of cancer survival. © 2007 Wiley-Liss, Inc.