This invention relates to novel thioflavin derivatives of the structure useful in, for example, in vivo imaging of patients having neuritic plaques, as well as pharmaceutical compositions comprising the thioflavin derivatives and method for synthesizing the compounds. The compounds find particular use in the diagnosis and treatment of patients having diseases where accum ulation of neuritic plaques are prevalent. The Disease, familial Alzheimers Disease, Downs Syndrome and homozygotes for the apolipoprotein E4 allele.