A word familiarity database (Amano & Kondo, 1999) was developed for about 70,000 spoken words to provide a basis for a psycholinguistic research in Japanese. Using this database, analyses and experiments were conducted on two lexical competitor sets, the neighborhood and the cohort, to examine their validity in lexical processings of Japanese spoken words. Firstly, the characteristics of the neighborhood and the cohort were analyzed in terms of their descriptive variables such as density (i.e., size), mean familiarity, maximum familiarity, sum of familiarity, and the uniqueness point as a function of a word familiarity. Correlation between the descriptive variables were also analyzed. Secondly, to investigate effects of the neighborhood and the cohort on Japanese spoken word recognition, a lexical decision experiment and a word recognition experiment were conducted using two sets of 100 spoken words which were selected from the database. The familiarity scores of the word stimuli ranged between middle (4.0) and high (7.0). These words and 100 nonwords were randomly presented to 30 Japanese subjects through headphones in the lexical decision experiment. The subjects were instructed to press one of two keys as soon as possible according to their judgment of whether they heard a word or a nonword. Reaction times were measured from the beginning of the stimulus. Partial correlation analyses between there action time and their descriptive variables excluding a factor of target word familiarity showed that there are no significant neighborhood effects or cohort effects on the reaction time. In the word recognition experiment, the words were randomly presented to 40Japanese subjects without noise or with noise at signal-to-noise ratios of -5, -2.5, 0, and 2.5 dB through headphones. The subjects were instructed to type what they heard using a computer keyboard. Recognition scores were obtained by dividing the number of correct answers by the total number of answers. Partial correlation analyses showed that there are significant neighborhood effects but no cohort effects on recognition scores. The results of two experiments showed that only the neighborhood affects the spoken word recognition in Japanese at least in a kind of 'unspeeded' task. It is suggested that the neighborhood is more plausible than the cohort as a lexical competitor set, and that some amount of time is necessary for the neighborhood to be activated and effective in lexical processing of spoken word recognition.