The purpose of this study was to determine if children acquire grammatical case as a unified system or in a piecemeal fashion. In English language acquisition, many children make developmental errors in marking case on subject position pronouns (e.g., "Me" do it, "Him" like it). It is unknown whether children who produce pronoun case errors with first person pronouns also produce errors with third person pronouns. This finding would be expected if case were acquired uniformly across person through building a paradigm for an abstract case feature. Spontaneous pronoun case errors were collected from language samples of 43 typically developing toddlers at 21, 24, 27, 30, 33, and 36 months of age. A chi-square test was used to determine whether children were more likely to make both first and third person errors, indicating an association. The uniformity of the case marking system was further investigated by asking whether pronoun case errors in first and third person occurred at the same time using a Wilcoxon signed-ranks test. Most children treated case uniformly across person, producing both first and third person pronoun case errors or producing no case errors at all, resulting in a significant association. Additionally, errors were not significantly different in timing. The results of this investigation are compatible with the notion that children systematically extend case marking in a unified paradigm. Pronoun case is not acquired separately for each grammatical person in a piecemeal fashion.