Bowen's Disease and Internal Malignant Disease

To the Editor.—  We want to thank Dr Jeffrey P. Callen for his valuable comments in his editorial "Bowen's Disease and Internal Malignant Disease," which appeared in the Archives.¹Callen discusses the introduction of biases when performing disease-association studies. He first mentions the diagnostic suspicion bias, which occurs when patients are scrutinized more carefully for a putative second disease. We would like to point out that the first part of our investigation (from 1943 to 1959) included patients who were studied before the publication by Graham and Helvig,² and that we, at all times during the study period, had not been convinced that Bowen's disease is a skin marker for internal malignant disease. Therefore, our patients were not specifically scrutinized as a consequence of having the diagnosis of Bowen's disease.Second, Callen mentions withdrawal bias and specifically states that "in the study by Reymann et al, there was