Objectives: To identify patients with retinoblastoma whose conditions were diagnosed at the age of 1 month or younger and to describe their clinical features (including ocular and patient survival) and the development of second nonocular tumors. Materials and Methods: A retrospective study of 1831 patients. The cumulative incidence of second cancer development was analyzed using the Kaplan-Meier method. Results: Forty-six patients were identified as having a diagnosis of retinoblastoma at the age of 1 month or younger (mean age, 18.5 days). Family history (31 patients [67%]) exceeded leukocoria (6 patients [13%]) as the most common reason for detection. Twenty-six (56%) of the 46 patients were seen with unilateral retinoblastoma, with 22 ultimately developing cancer in the fellow eye. At the initial diagnosis, 81 (85%) of the 95 tumors were detected in zones 1 and 2. Eighty-two (93%) of the 88 subsequent tumors were located in zones 2 and 3. In the 26 patients who had unilateral retinoblastoma, 16 of the initially affected eyes and 21 of the fellow eyes were salvaged. In the 19 (44%) of 20 patients who were seen initially with bilateral retinoblastomas, 31 (82%) of the 38 eyes were salvaged. The mean follow-up was 10.9 years. The incidence of second nonocular cancers reached 54% by 23.7 years for the patients who received radiation therapy, while the incidence was 0% for the patients who did not. Four(8.7%) of the 46 patients developed metastatic disease and died; 3 of these patients had documented metastases in the first month of life (one at birth). Conclusions: The most common manifesting sign of children diagnosed as having retinoblastoma in the first month of life is family history. Eyes with Reese-Ellsworth group I retinoblastomas were the most common. In patients with bilateral and unilateral retinoblastoma, new (subsequent) ocular tumors developed in a centrifugal pattern. Despite an early diagnosis, patients' eyes came to enucleation, and metastatic disease and death occurred from ocular metastases. In patients who received radiation therapy, the probability of developing second nonocular cancer is 54% by 23.7 years; no second cancers developed in patients who did not receive radiation therapy. Arch Ophthalmol. 2002;120:738-742