OBJECTIVE The risk of women whose chief complaint is recurrent spontaneous abortions (RSA) for secondary infertility (infecundability) has not been evaluated prospectively. The effect of paternal mononuclear cell immunization on conception rates is unknown. METHODS Two hundred women whose chief complaint was RSA were randomly assigned to be immunized with paternal mononuclear cells either before or after (up to 6 postmenstrual weeks) conception. Fertility rates (both conception and live birth) were evaluated for the group immunized before conception and compared to those for the control group, who were not immunized until after conception, using life table and multiple logistic regression analyses. RESULTS Prospectively ascertained, age-related conception rates for nonimmunized RSA controls appeared to be similar to those for general populations. Immunization before pregnancy had no significant effect (power +/- 14%) on rates of conception (66% before, 77% after) or time to conceive (median weeks before 19.5, after 27.0). Live birth rates (before 59%, after 63%) were also similar for both groups (P = 0.7). CONCLUSIONS Women whose only prior complaint was RSA were not at high risk for secondary infecundability, and immunization did not alter either conception rates or time to conceive. Postponement of immunization until after conception did not affect live birth rates for women selected for study because they did not have a history of prior infecundability or early repeated miscarriages.