Objective: When we review the current literature on endometriosis and luteinized unruptured follicle (LUF), we see that most of the studies deal with only the association between LUF frequency and the severity of endometriosis. Our purpose was to evaluate the effect of ovarian involvement on LUF frequency in endometriosis and assess the relationship between endometriosis and LUF in infertile women. Study Design: This study is a prospective analysis covering a total of 126 infertile women between 22 and 35 years of age who underwent diagnostic laparoscopy from September 1995 to August 1997 in the Department of Obstetrics and Gynecology at Suleyman Demirel University, Isparta, Turkey. Endometriosis was diagnosed in 58 of these patients. All had received a revised American Fertility Society staging score at the time of the laparoscopic diagnosis. Diagnosis of LUF was made when the following criteria were fulfilled: absence of ultrasonic signs of ovulation, and absence of an ovulation ostium on the follicle by laparoscopy, despite increased serum progesterone. Statistical evaluation was performed using chi(2) test and Fisher's exact test where appropriate. Results: The prevalence of LUF in mild, moderate and severe endometriosis cases was 13.3, 41.2 and 72.7%, respectively. The LUF frequencies were 45.9% in 37 endometriosis patients with ovarian involvement, 9.5% in 21 cases without ovarian involvement and 5.9% in 68 cases without endometriosis. A statistically significant difference was observed between the LUF rate in the group with ovarian involvement and that without involvement (chi(2) = 8.06, p < 0.001). Conclusion: In summary, in this study we noted a significant increase in LUF frequency in endometriosis patients with ovarian involvement.