When very large groups have to brainstorm in smaller subgroups, meeting organizers have a choice between two options: (1) Let every subgroup start from scratch, or (2) Let every next subgroup build on the results from the previous subgroup. We refer to these options as the Decathlon and Relay modes of brainstorming respectively. During a case study we investigated which brainstorming mode would be more productive and result in higher levels of participant satisfaction. Consistent with the hypotheses, Relay groups appeared to be more productive than Decathlon groups, in particular in terms of elaborations to previous contributions. Relay groups were also found to be more satisfied. This study also introduced the elaboration coefficient, an indicator to quantify the amount of task relevant discussion in brainstorming groups. The results of this study hint at a number of meeting design guidelines for very large brainstorming groups.