The effect of application time of CPP-ACPF on enamel hardness. Remineralization process can increase the hardness of enamel due to demineralization process. CPP-ACPF is a material used for enhancing remineralization. However, the application time of CPP-ACPF remain controversial among previous studies. This study was aimed to investigate the effect of various application times of CPP-ACPF on enamel hardness. Thirty premolar teeth were mounted on self cure acrylic resin, and were divided into 5 groups. Demineralization process was performed, and enamel hardness (pre-est) was measured by Vickers Hardness Tester. Remineralization was performed using CPP-ACPF in various application times: 3, 15, 30, 60 minutes, and the control group was only immersed in artificial saliva for 60 minutes, then enamel hardness was measured (posttest). Data were analyzed using paired t-test, one-way ANOVA, and post-hoc Bonferroni. The result of paired t-test showed that all the groups, except the control group, have an increasing enamel hardness that was statistically significant. One-way ANOVA results showed no statistically significant difference among the groups at pretest, but one-way ANOVA results showed statistically significant difference at posttest. Post hoc Bonferroni showed that the significantly difference at posttest occurred between all the treatment groups against the control group, but there were no significant differences between the 3 minutes group to 15 minutes group, between 15 minutes group to 30 minutes group, and between 30 minutes group to 60 minutes group. It was concluded that various application times of CPP-ACPF had an effect on increasing enamel hardness. ABSTRAKProses remineralisasi dapat meningkatkan kekerasan email yang menurun akibat demineralisasi. Bahan remineralisasi yang ideal adalah CPP-ACPF. Terdapat perbedaan lama aplikasi CPP-ACPF dalam berbagai penelitian, selain itu total lama aplikasi yang dibutuhkan CPP-ACPF dalam mekanisme remineralisasi belum diketahui. Penelitian ini dilakukan untuk mengetahui pengaruh lama aplikasi CPP-ACPF terhadap kekerasan email. Tiga puluh mahkota gigi premolar yang ditanam dalam resin akrilik self cure dibagi menjadi lima kelompok, kemudian dilakukan proses demineralisasi. Kekerasan email kemudian diukur menggunakan alat Vickers Hardness Tester. Proses remineralisasi menggunakan CPP-ACPF dilakukan pada masing-masing kelompok dalam berbagai lama aplikasi yaitu 3 menit, 15 menit, 30 menit, 60 menit, serta perendaman dalam saliva buatan selama 60 menit (kontrol). Kekerasan email kemudian diukur kembali (posttest). Data diuji secara statistik menggunakan t-test berpasangan, one-way ANOVA dan post hoc Bonferroni. Hasil paired t-test menunjukkan bahwa seluruh kelompok, kecuali kelompok kontrol, mengalami peningkatan rata-rata kekerasan email secara signifikan. Hasil uji one-way ANOVA pada pretest menunjukkan tidak ada perbedaan kekerasan email yang signifikan. Hasil uji one-way ANOVA pada posttest menunjukkan terdapat perbedaan kekerasan email yang signifikan. Hasil uji post Hoc Bonferroni menunjukkan bahwa terdapat perbedaan kekerasan email yang signifikan pada seluruh kelompok perlakuan terhadap kelompok kontrol, tetapi perbedaan kekerasan email antara kelompok lama aplikasi 3 menit dengan 15 menit, antara lama aplikasi 15 menit dengan 30 menit, serta antara lama aplikasi 30 menit dengan 60 menit tidak menunjukkan perbedaan kekerasan email yang signikan. Kesimpulan penelitian ini adalah berbagai lama aplikasi CPP-ACPF berpengaruh terhadap peningkatan kekerasan email.