The April 7, 1962 criticality accident involving human exposures was the first to have occurred in any production facility at Hanford. The accidental nuclear excursion did not result in any mechanical damage or spread of contamination. Three employees received over-exposure to gamma and neutron radiation. None were fatally exposed and in each case the over-exposure was recognized promptly. Following an initial period of medical observation and testing, the men were released to work. They continued to be followed clinically. Clinical studies performed were hematological procedures including leukocyte chromosome aberrations, morphologically aberrant blood cells, bone marrow evaluations, blood chemistry determinations, amino acid excretion studies, seminal fluid, urinary gonadotropins and estrogen excretion studies, testicular biopsies and crystalline lens examinations. These studies, along with a brief description of the accident and of the dosimetry, are summarized in this report by those participating in the studies. In view of the dose ranges received in these cases, both the negative and positive findings are considered to be of unusual interest due to the lack of knowledge of effects following human exposures at these levels.