This research is novel in its exploration of and insights gained about the associations of platelet 5-HT concentration, platelet MAO-B activity, and the SNPs of the HTR2A, HTR2C, and MAOB genes with adult asthma in an ethnically and racially homogeneous study population (i.e., Croatian Caucasians) of asthma patients and healthy control subjects. This evidence concerns the gene MAOB and asthma.