Quantitative and qualitative research writing is thought to differ in a number of ways, which include the visibility given to the human agents involved, that is, writers and participants in the study. However, most studies have so far centred on writer visibility alone, which has been measured principally through personal pronoun use. This paper approaches the issue of writer and participant visibility in one area of research where both quantitative and qualitative methods are frequent, namely health sciences. A new methodology is applied, based on the presence of verbs that imply a human agent and thus potentially offer an opportunity for writer or participant visibility. Two corpora are built to represent quantitative and qualitative research writing in health sciences, and quantitative data are obtained. Exploration of the most frequent human-agent verbs, their active uses and their subjects shows that both quantitative and qualitative researchers in health science maintain low writer visibility, but that both writers and participants are more visible in the case of studies using qualitative methodology. These findings are discussed in the light of the bibliography, and pointers are provided for future research. The contribution of the present paper to corpus-assisted research on academic writing is highlighted. Finally, some applications for teaching scientific writing at graduate level are proposed.