Purpose: The purpose of the present study was to evaluate and compare treatment with mandibular overdentures supported by one or two implants placed and loaded immediately. Materials and Methods: Edentulous subjects with maxillary and mandibular complete dentures were recruited to participate in the study. All subjects received new maxillary and mandibular complete dentures, after which they were scheduled for implant surgery. Using a randomized sampling system, one or two implants were placed in the interforaminal area, and separate ball attachments were connected to the implants. The existing mandibular denture was relined, provided with retentive o-rings, and delivered the day of surgery. Subjects were seen for follow-up examinations after 3, 6, and 12 months, during which marginal bone levels and implant stability were measured. Survival rates were calculated and the data were analyzed statistically. Results: Thirty-six subjects (16 men and 20 women with a mean age of 53.2 years [range, 38 to 69 years]) received 55 implants. Seventeen subjects received one implant in the symphyseal area, and the other 19 received two implants in the canine positions. Ten implants in nine subjects failed during the observation period, and three subjects with two implants each withdrew from the study, resulting in a 12-month implant survival rate of 81.8%. Three subjects lost their only implant and one patient lost both implants. The remaining five subjects lost one of their two implants. Six of the failures occurred during the first month after placement. Conclusion: Immediate loading of one or two separate implants by means of ball attachment-supported mandibular overdentures should be performed with caution, as a higher than expected implant failure rate was observed. No statistically significant differences in failure rates between the two groups were observed. Keywords: dental implant, immediate loading, mandibular overdenture, osseointegration