Small-angle neutron scattering has been applied for structural investigations of amine-terminated poly(propylene imine) dendrimer (Astramol) solutions in D2O as a function of concentration and acidity. The forward scattering, d Sigma/d Omega(Q = 0), decreases progressively by increasing dendrimer concentration due to interdendrimer interference, giving rise to a well-defined interference peak. Upon increasing dendrimer concentration, a shift is observed in the peak position, Q(max), with the empirical scaling relation Q(max) similar to phi(0.55+/-0.05), which is significantly larger than that of a simple packing argument Q(max) similar to phi(1/3). At high Q values, all data sets collapse to a single curve, indicating that the internal structure of the dendrimers are unaffected by interactions between neighboring molecules. Upon addition of acid, the dendrimers become charged, giving rise to a pronounced interference peak. These Coulomb interactions can be screened by addition of either excess acid or salts.