Time-dependent nonlinear flow behavior was investigated for a model hard-sphere suspension, a 50 wt% suspension of spherical silica particles (radius = 40 nm; effective volume fraction = 0.53) in a 2.27/1 (wt/wt) ethylene glycol/glycerol mixture. The suspension had two stress components, the Brownian stress sigma(B) and the hydrodynamic stress sigma(H). After start-up of flow at various shear rates y(over dot), the viscosity growth function eta(+) (t,y(over dot)) was measured with time t until it reached the steady state. The viscosity decay function eta(-) (t,y(over dot)) was measured after cessation of flow from the steady as well as transient states. At low y(over dot) where the steady state viscosity eta(y(over dot)) exhibited the shear-thinning, the eta(-) (t,y(over dot)) and n(+) (t,y(over dot)) data were quantitatively described with a BKZ constitutive equation utilizing data for nonlinear relaxation moduli G (t,y). This result enabled us to attribute the thinning behavior to the decrease of the Brownian contribution eta(B) = sigma(B)/y(over dot) (considered in the BKZ equation through damping of G (t,y)). On the other hand, at high y(over dot) where eta (y(over dot)) exhibited the thickening, the BKZ prediction largely deviated from the eta(+) (t,y(over dot)) and eta(-) (t,y(over dot)) data, the latter obtained after cessation of steady flow. This result suggested that the thickening was-due to an enhancement of the hydrodynamic contribution eta(H) = sigma(H)/y(over dot) (not considered in the BKZ equation). However, when the flow was stopped at the transient state and only a small strain (<0.2) was applied, eta(H) was hardly enhanced and the eta(-)(t,y(over dot)) data agreed with the BKZ prediction. Correspondingly, the onset of thickening of eta(+) (t,y(over dot)) was characterized with a y(over dot)-insensitive strain (congruent to 0.2). On the basis of these results, the enhancement of eta(H) (thickening mechanism) was related to dynamic clustering of the particles that took place only when the strain applied through the fast flow was larger than a characteristic strain necessary for close approach/collision of the particles.