The molecular weight and molecular weight distribution of isotactic polyacrylonitrile (PAN) prepared by urea clathrate polymerization in the solid state were studied by the g.p.c.-low-angle laser light scattering (l.a.l.l.s.) method. The experimental details of this method for PAN are described. Measurements were carried out on three different kinds of urea clathrate PAN, and the results were compared with those of free-radical PAN (i.e. prepared using the aqueous redox slurry system). The characteristics of the urea clathrate PAN were (1) an asymmetrical shape in the g.p.c.-l.a.l.l.s. curve and (2) a remarkable contribution of the lower-molecular-weight component to the high stereoregularity (>80%) of the sample. An experimental prediction for the solid-state synthesis of a perfectly stereoregular PAN sample (isotacticity of 100%) has been derived from a combination of the g.p.c.-l.a.l.l.s. and C-13 n.m.r. results.