The main risk factor of post-ivermectin serious adverse events (SAEs) is the presence of a high Loa loa microfilaremia. However, the majority of patients with such high loads do not develop SAEs, suggesting that co-factors may be involved. An infection with simian Loa parasites, whose microfilariae show a nocturnal periodicity, might be such a co-factor. The periodicity of Loa microfilariae was compared, using cosinor methodology, in 4 patients who had developed a post-ivermectin neurologic SAE, 4 patients who had experienced a non-neurologic SAE, and 14 control individuals. The periodicity was similar in all three groups, with a peak of microfilaremia occurring between 12:30 and 2:00 PM. The results of this study, which for the first time characterizes the periodicity of Loa microfilariae mathematically, suggest that post-ivermectin SAEs are not related to an infection with a Loa simian strain.