Purpose - This paper aims to extend understanding of business-to-business relationship dynamics by introducing and discussing the phenomenon of a "negative critical wave" (NCW), defined as a disturbance in a relationship that emerges and develops within or beyond individual working relationships. Design/methodology/approach - The dynamics of working relationships in two manufacturing firms in Finland and Sweden were studied by analysing the narratives of unstructured personal interviews with 16 middle managers and 14 operational executives, who recalled experiences of relevant situations over three years, with emphasis on unexpected disturbances, challenges and problems. Findings - Respondents discussed 77 NCWs, the development and effect of which proved to depend upon the original "locus", "magnitude" and "amplitude", and embedded "energy". Waves could be distinguished as: "silent compact", "silent extensive", "intense compact" or "intense extensive". Research limitations/implications - The wave metaphor for relationships dynamics, consistent with but distinct from established notions of "critical time" and "critical incidents" and the associated classification system are a useful starting point for further research into the phenomenon. Though the qualitative methodology achieved richness, the small sample and restricted scope place limits on the objectivity and generalisability of the findings. Practical implications - The NCW framework offers strategists and managers a holistic understanding of the dynamic process of criticality, synthesising the complexities of relationship dynamics and pointing to ways in which to absorb the energy of negative waves. Originality/value - More is now known about the domino effects of critical incidents in internal and external business-to-business relationships.