Serial binary dilutions of amyl acetate and nitrobenzene were used in a double-blind test to determine olfactory acuity of 40 multiple sclerosis (MS) patients. Similar determinations were made on 24 age- and sex-matched controls. Results of repeated measurements on 18 patients indicated that the methodology was reliably reproducible. When mean olfactory threshold on MS patients was compared with that of the controls no differences could be found. Among the MS patients, olfactory threshold did not vary with the presence or absence of optic involvement and correlated poorly with the degree of disability. These results are in disagreement with those of Wender and Szmeja who reported olfactory dysfunction in 18 out of 52 MS patients. Our findings are compatible with the observations of Zimmerman and Netsky who failed to find any plaques in olfactory tracts of MS brains. The significance of this disparity between involvement of optic and olfactory systems, both of which contain central myelin, is discussed. Available biochemical evidence suggests that, as postulated by Lumsden, sparing of olfactory tracts may in part be related to the relatively lower basic protein content of these structures.