In order to determine the true incidence of treatment-dependent versus -independent pregnancy in an in vitro fertilization (IVF) program, 274 women who underwent 492 cycles of superovulation were studied. Overall, the treatment-dependent pregnancy rate was 15%. The treatment-independent pregnancy rate was 6.6%. When a subgroup of individuals with at least one patent fallopian tube was selected for analysis, the treatment-dependent and -independent pregnancy rates were 13.9% and 11.9%, respectively. While the mean observation interval following an attempt at IVF was 2 years, 83.3% of all treatment-independent pregnancies occurred within 6 months after a trial of IVF-ET (embryo transfer). Patient characteristics that predispose to treatment-independent pregnancy are discussed.