Electrospray mass spectrometry (ESI-MS) is used to analyze in detail the structure of the first five generations of the poly(propylene imine) dendrimers with 4-64 end groups. Although the reaction sequence to prepare these dendrimers is highly optimized, an accumulation of statistical defects is observed at higher generations. A simple statistical simulation allows the assignment of all peaks in the deconvoluted spectrum and the determination of yields for each side reaction in the formation of each generation. The fifth generation of the poly(propylene imine) dendrimer containing amine end groups, which is formed after 248 consecutive reaction steps, possesses a dendritic purity of approx. 20%, which corresponds to a polydispersity of approximately 1.002. The defects in the divergent growth patterns of dendrimers are compared to those observed in the Merrifield synthesis of polypeptides.