Little is known about the influence of language background in problem-based learning (PBL) tutorial groups on group processes and students' academic achievement. This study investigated the relationship between language background, secondary school score, tutorial group processes, and students' academic achievement in PBL. A validated tutorial group effectiveness questionnaire was administered to undergraduate medical students in a PBL curriculum at the Nelson R. Mandela School of Medicine (NRMSM) in South Africa. Although 58 percent of the students did not speak English as their first language, the tutorials were in English. Furthermore, secondary school scores differed strongly due to inadequate resources between secondary schools. A path analysis was conducted to test a causal model in which the two independent variables were English as the First Language (EFL) and secondary school scores. These variables were assumed to influence the process variables (cognitive, motivational, and demotivational group processes). Input and process variables were assumed to influence the two output variables, being overall group productivity and students' academic achievement. All data were analyzed at the individual student level (N = 387). A very good model fit was found (CMIN/DF = 0.68, GFI = 1.00, TLI = 1.02, CFI = 1.00, RMSEA = 0.00). EFL and secondary school scores positively affected students' academic achievement (respectively beta = 0.24 and beta = 0.16). EFL negatively affected motivational group processes (beta = -0.22). Cognitive group processes positively influenced overall group productivity (beta = 0.31) and so did motivational group processes (beta = 0.27). Demotivational group processes negatively predicted academic achievement and overall group productivity (beta = -0.15, and beta = -0.25). The model resulted in an R-square of 0.15 and 0.45 for academic achievement and overall group productivity, respectively. EFL and secondary school scores had a positive effect on academic achievement. Cognitive and motivational group processes had a positive effect on overall group productivity, while EFL negatively impacted motivational group processes. We recommend English language development courses to be formally included into curricula to enhance student learning.