Objective-To examine 12-month effects of a booster-enhanced preconception counseling (PC) program (READY-Girls) on family planning for teen girls with type 1 and type 2 diabetes. Research design and methods-Participants 13-19 years of age (n = 109) were randomized to a standard care control group (CG) or intervention group (IG) that received PC over three consecutive clinic visits. Prepost data were collected at baseline, 3- and 6-month booster sessions, and a 12-month follow-up visit. Results-Mean age was 15.8 years; 9 (8%) subjects had type 2 diabetes; and 18 (17%) subjects were African American. At baseline, 20%(n = 22 of 109) had been sexually active, and of these, 50% (n = 11) had at least one episode of unprotected sex. Over time, IG participants retained greater PC knowledge (F[6, 541] = 4.05, P = 0.0005) and stronger intentions regarding PC (significant groupby-time effects) especially after boosters. IG participants had greater intentions to discuss PC (F[6, 82.4] = 2.56, P = 0.0254) and BC (F[6, 534] = 3.40, P = 0.0027) with health care providers (HCPs) and seek PC when planning a pregnancy (F[6, 534] = 2.58, P = 0.0180). Although not significant, IG participants, compared with CG, showed a consistent trend toward lower rates of overall sexual activity over time: less sexual debut (35 vs. 41%) and higher rates of abstinence (44 vs. 32%). No pregnancies were reported in either group throughout the study. Conclusions-READY-Girls appeared to have long-termsustaining effects on PC knowledge, beliefs, and intentions to initiate discussion with HCPs that could improve reproductive health behaviors and outcomes. Strong boosters and providing PC at each clinic visit could play important roles in sustaining long-term effects.