The 'epidemic' of asthma deaths observed in the mid 1960s remains unexplained; various aetiological factors having been suggested. We suggest that aero-allergenic factors may have been important based on an analysis of aero-allergens in Cardiff over a 17-year period: a striking temporal association exists between the concentration of hyaline ascospores and increased asthma deaths. These spores, which are particularly prevalent in the summer months, have been inadequately studied as a cause of severe asthma.