Interprofessional education (IPE) for health and social care students may improve attitudes toward IPE and interprofessional collaboration (IPC). The quality of research on the association between IPE and attitudes is mediocre and IPE effect sizes are unknown. Students at a college in Toronto, Canada, attended an IPE workshop. A comparison group of non-attenders was formed. Both groups completed pre- and post-workshop questionnaires and two measurement scales for IPE attitudes-the Interprofessional Education Perceptions Scale and the University of West England questionnaire. Eight multiple linear regressions modeled post-workshop attitude scores as a function of workshop attendance, pre-workshop attitudes, and background factors. Workshop effect sizes and relative importance of variables were estimated. Published results were used to calculate IPE effect sizes in other studies. Pre-workshop measures of post-workshop attitudes were dominant, positive predictors of outcomes; other predictors were subordinate to them. The relationship between workshop attendance and IPE attitudes was positive in seven models, statistically significant in four (P < 0.05), and not statistically significant in four. In hierarchical regressions the average workshop effect was small, about 0.08 in ΔR(2) terms, amounting to about one-quarter of a model's explained variance. The workshop was associated with improved IPE and IPC attitudes in some domains but not others. The results should help educators focus IPE efforts on IPE/IPC attitudes where small, short-term improvements can be obtained.