Background: Over the past twenty years, many investigations have been carried out to identify factors influencing second language (L2) learning. Specifically, investigations of the relationship among first language (L1) reading skills, L2 reading skills, and L2 proficiency have been variously conducted, to contribute to the overall growth of L1-L2 relation research. By contrast, less emphasis has been placed on research concerning the interrelationship among L1 writing skills, L2 writing skills, and L2 proficiency. Because studies of the interrelationship are still in the beginning stages, there cannot yet be consensus on research findings. Purpose: To examine the interrelationship among L1 writing skills, L2 writing skills, and L2 proficiency of Japanese EFL (English as a Foreign Language) college students. Setting: A Japanese four-year university in 2002. Study Sample: 262 undergraduate students participated as subjects in the present study. They were non-experienced Japanese EFL writers of academic English essays who had had at least six years of English education provided in secondary schools in Japan. Research Design: Correlational. Control or Comparison Condition: Data Collection and Analysis: The TOEIC (Test of English as International Communication) was administered in January 2002 at the first session. One week after this TOEIC administration, the second session was conducted, which consisted of the first writing tests (137 students composed L1 essays; 125 students composed L2 essays). The third session consisted of the second writing tests (137 students composed L2 essays; 125 students composed L1 essays). The researcher performed a correlational analysis among the three criteria measures (the Japanese composition, the English composition, and the TOEIC scores) using a Pearson product-moment correlation procedure. Findings: There is an observable interrelationship among L1 writing skills, L2 writing skills, and L2 proficiency. At the 0.01 level, the correlations of 0.477 and 0.389 are significant between Japanese writing and English writing scores, and between the TOEIC and English writing scores. Moreover, a weak but significant correlation (r = 0.233) at the 0.01 level between the L1 writing and the TOEIC scores is exhibited in this study. Conclusion: The better quality of students' L1 writing appears to bring about better performance in L2 writing. L2 linguistic proficiency seems to facilitate the overall quality of L2 writing products. Although the link between L1 essay writing skills and L2 proficiency is still only speculative because of the relatively low correlation, L1 writing proficiency seems to be related to L2 proficiency. (Contains 6 tables and 2 figures.)