Expressions are derived for the relative r.m.s. errorσ of diffractometer intensity measurements. The result for stationary specimens: $$\sigma = 4R[\sin \theta /m w h N_{eff} ]^{\tfrac{1}{2}} $$ withh=1/2(h F+h S) and $$N_{eff} = cA\bar v/\mu v^2 $$ , is identical with the result of Alexander c.s.1), except for a slight difference in the numerical constant and in the definition ofw. The value of this parameter is found to lie betweenɛR+(w F, wS) min (the last term indicating the smallest of the widthsw F andw S) andɛR+1/2(w F+w F); it reaches the latter limit in the case of integrated intensities being measured by totalizing counts while scanning through a line. For rotating specimens the particle statistics error turns out to be almost independent ofw. The following approximative formula is established:σ=6.5R sinΘ/h(mN eff)1/2, showing that the factor of improvement resulting from specimen rotation is of the order of (h/w)1/2.