ABSTRAKLatar Belakang : Masa remaja lanjut (17-21 tahun) terutama perempuancenderung mengkonsumsi makanan tanpa lemak untuk mengutamakan kecantikantubuhnya, namun tidak sedikit dari mereka makan secara berlebihan sehinggamengalami obesitas. Status gizi yang di ukur dengan indeks massa tubuh dapatmempengaruhi sitem hormon dalam tubunya, sehingga berpengaruh pada siklusmenstruasinya. Tujuan penelitian ini untuk mengetahui hubungan indeks massatubuh dengan lama siklus menstruasi mahasiswa D III Kebidanan FK UNS.Metode : Desain penelitian analitik observasional dengan pendekatan crosssectional. Jumlah sampel 80 responden yang telah di seleksi menggunakan LMMPIdan teknik purposive sampling. Uji statistik menggunakan spearmanrank.pada ? 5 %.Hasil : Sebagian besar indeks massa tubuh responden dalam kategori normal(67,5%) dan sebagian kecil dalam kategori gemuk (7,5%). Sebagian besar lamasiklus menstruasi responden dalam kategori normal (21-35 hari) (60 %). Hasil ujistatistik didapatkan nilai p = 0,002 dan r = 0,343.Simpulan : Ada hubungan Indeks Massa Tubuh dengan Lama Siklus MenstruasiMahasiswa D III Kebidanan FK UNSKata Kunci : Indeks Massa Tubuh, Lama Siklus Menstruasi, MahasiswaKebidananABSTRACTBackground: In post-adolescence (17-21 years), the females particularly tend toconsume fat-free food intentionally so as to bear physical beauty, but not few ofthem eat such food excessively so that they become obese. Nutrient status thatmeasured by body mass index, can affect their body’s hormonal system, whichlater may affect their menstrual cycle. The purpose in this researh is to knowcorrelation between Body Mass Index and menstrual cycle length of the studentsof Diploma III in midwifery science, the Faculty of Medicine, Sebelas MaretUniversity.Method: This research used the observational analytical research method withthe cross- sectional approach. Its samples was 80 respondent who selected with LMMPIand purposive sampling technique. The data of research were statisticallyanalyzed by using the Spearman’s Rank Test with ? 5 %.Result: Most of the respondent had normal body mass index (67,5%) and a smallportion of the respondent had obese body mass index (7,5%). Most of therespondent had normal menstrual cycle length (21-35 days) (60 %). The statisticaltest shows that the p-value = 0.002 and r = 0.343.Conclusion: There was a correlation between the body mass index and themenstrual cycle length of the students of Diploma III in Midwifery Science, theFaculty of Medicine, Sebelas Maret University.Keywords: Body mass index, menstrual cycle length, midwifery students