The purpose of the study was to find how nationality, sex, and past experience of seeking professional psychological services are related to attitudes toward seeking professional psychological help. Three hundred Japanese college students and 300 US college students responded to the Attitudes Toward Seeking Professional Psychological Help (ATSPPH) questionnaire. The ATSPPH consists of four subscales: Need (recognition of need), Stigma Tolerance (the degree of tolerance against stigma associated with help-seeking action), Openness (interpersonal openness), and Confidence (confidence in mental health professionals). As predicted, past experience of seeking professional psychological service and sex were important predictor variables of performance on the ATSPPH scales. Those who had past experience of seeking professional psychological help had more favourable attitudes toward seeking professional psychological help than those who never consulted psychological professionals. Similarly, in testing the past experience separately with the two nation groups, past experience with seeking professional psychological services was found to be a predictor of the overall attitudes toward seeking professional psychological help among Japanese and US participants. Furthermore, Japanese participants with past experience also showed greater recognition of need for professional psychological help and confidence in psychological health professions than Japanese without past experience. The degree of stigma tolerance associated with help-seeking behaviour and the tendency of interpersonal openness, however, did not differ between Japanese participants with and without the past experience of seeking professional psychological services. The same results were also found among the US participants. In terms of sex as a predictor variable, females tended to have more positive attitudes toward seeking professional psychological help than males. There was a significant nationality by sex interaction effect. US females had significantly more favourable attitudes than the other three groups, indicating that there was sex difference in help-seeking attitudes in the US group, but not in the Japanese group. If the sex variable is ignored, Japanese students have less favourable overall attitudes toward seeking professional psychological services than US students. Other results and limitations of the study are discussed.