During a 3-week eruption in 1984, Mauna Loa produced vent lavas that increased in crystallinity from <1 to 30%, and 27-km-long flows that increased in crystallinity as they moved downstream. We examined the crystallization history of these lavas using crystal size distribution (CSD) analysis to study the rates of crystallization, viscosity increase, and latent heating. Typical average growth and nucleation rates were 5¿10-9 cm s-1 and 5 cm-3 s-1 for microphenocrysts (20- to 500-μm size crystals nucleated in the rift zone) and 5¿10-8 cm s-1 and 5¿104 cm3 s-1 for microlites (1- to 20-μm size crystals nucleated in the channel). These crystallization rates are high compared with those found in other CSD studies of igneous rocks, probably due to highly nonequilibrium conditions brought on by rapid degassing in the rift zone and cooling in the lava channel. Growth and nucleation rates decreased with time at the vent and with distance downstream. The maximum downstream total crystallinity measured is 39% (25% microlites, 14% microphenocrysts) in a quenched sample 14 km from the vent. Growth and nucleation rates cannot be calculated for postemplacement samples, but they place upper limits of 53--58% on the amount of crystallization in the channel 9--20 km from the vent. Crystallization could have been mostly responsible for the 105-fold downstream increase in apparent viscosity, although degassing and increasing incorporation of solid lava fragments also contributed. Another effect of crystallization on the lava flow was the sizeable latent heating (0.01 J g-1 s-1 over the first half of the flow length, if the crystallinity of downstream quench samples is representative of the hot fluid core), which may have been counteracted by entrainment of cooler material. Measurements of crystallization are shown to be crucial in the study of lava flow emplacement dynamics.