Theories of autobiographical memory distinguish between involuntary and voluntary memories. While involuntary memories are retrieved with no conscious intention and are therefore unexpected, voluntary memories are both intended and expected. Recent research has shown that participants sometimes classify their memories as either involuntary or voluntary based on retrieval effort instead of intention. These findings question whether intention really is the defining difference between these two types of retrieval or whether retrieval effort is also an important determinant. In two experimental studies, we investigated the extent to which participants rely on retrieval effort while classifying their memories as involuntary or voluntary. We created experimental conditions that maximize the probability of one type of retrieval while minimizing the probability of another type. Participants reported autobiographical memories in each condition while the programme registered their retrieval time. They then classified their memories as either voluntary or involuntary and rated all memories on perceived retrieval effort. This gave us four categories of memories: experimentally defined voluntary and involuntary memories with an objective measure of effort (retrieval time) and subjectively classified involuntary and voluntary memories with a subjective measure of effort. This allowed us to investigate the relative contribution of intention and effort to involuntary and voluntary memories. We replicated and extended previous findings by showing that the majority of memories were classified as involuntary independently of whether they were retrieved in the experimentally defined voluntary or involuntary condition. This could indicate that subjective effort is more important than intention for the involuntary–voluntary distinction in contrast with existing theories. We discuss theoretical and methodological implications of this finding.