There has been an increasing interest in lesbian, gay, bisexual, transgender, intersex and queer (LGBTIQ) issues in social science research in Croatia. Additionally, LGBTIQ people can be sampled in almost any research. However, it is usually assumed that all study participants are heterosexual and cisgender. In consequence, a limited number of categories for LGBTIQ individuals are typically included in questionnaires. Such an approach results in biased sample characteristics, which can both affect the interpretation of results and provoke LGBTIQ participants to refrain from providing answers to certain questions or refuse further participation. In addition, it represents a violation of the scientific research ethical principle of avoiding discrimination or privilege based on sex, gender identity, and expression or sexual orientation. In this paper, recommendations are offered for research that acknowledges LGBTIQ people by including different response options for sexual orientation, sex, and gender identity. These recommendations are based on an overview of different theoretical explanations and approaches in the measurement of gender, sex and sexual orientation in Croatian and international research, as well as on the authors’ experience in professional work with LGBTIQ people. As a possible step towards recognising LGBTIQ identities, introducing questions about sexual orientation and gender identity into research is recommended where those variables could be relevant for the studied subject. Also, since sex is a frequent research variable and commonly used response options “male” and “female” do not apply to a part of the population, adding a response option for self-identification or “something else” is recommended. In addition to item construction examples, explanations of the key concepts important for research including LGBTIQ people are provided.