The Food and Drug Administration issued on February 9, 2012, drafts of three new guidance documents about the demonstration of biosimilarity. One of these deals with scientific considerations. It suggests, among others, that demonstration of biosimilarity be developed by a stepwise (step-by-step) approach and that it be assessed by considering the totality of the evidence. This communication provides comments on some scientific factors and issues that still remain unanswered or unsolved. They include the question ‘how similar is considered to be highly similar?’ considerations of criteria for and the degree of biosimilarity; alternatives of study design and sample size requirements; statistical methods for achieving the totality of the evidence needed for biosimilarity; and methods needed for the assessment of drug interchangeability. It is anticipated that the comments will assist the revision of the guidance documents. Copyright © 2012 John Wiley & Sons, Ltd.